As in topic - i changed the 1.4.0 mod to 1.5.0 and server dont want to start. There is no crash log either.
I had similar problem with slimier slimes just a day ago so i think it can be fixed by little update.
This one not affecting single player (works fine) - just starting server
Please check if possible

The mod works fine on a dedicated server for me. Strange that there is no log, that makes debugging harder. Were you using a custom data-pack for spells before the update?

Edit: If you look in the logs folder where your server jar is located, are you sure that there is not a file named 'debug.log' that is updated when the server fails to start?

Yes i have that one.
And yes i had a data pack either. Should i remove it?

Caused by: com.google.gson.JsonSyntaxException: Missing blacklist, expected to find a JsonArray
at net.minecraft.util.JSONUtils.func_151214_t(SourceFile:425) ~[?:?]
at mod.vemerion.wizardstaff.Magic.original.EggMagic.readAdditional(EggMagic.java:36) ~[wizard-staff:1.16.5-1.5.0]
at mod.vemerion.wizardstaff.Magic.Magic.read(Magic.java:54) ~[wizard-staff:1.16.5-1.5.0]
at mod.vemerion.wizardstaff.Magic.Magics.apply(Magics.java:179) ~[wizard-staff:1.16.5-1.5.0]
at mod.vemerion.wizardstaff.Magic.Magics.func_212853_a_(Magics.java:67) ~[wizard-staff:1.16.5-1.5.0]
at net.minecraft.client.resources.ReloadListener.func_215269_a(SourceFile:13) ~[?:?]
at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:670) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:646) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:456) ~[?:1.8.0_282]
at net.minecraft.resources.AsyncReloader.func_219557_a(SourceFile:71) ~[?:?]
at net.minecraft.resources.AsyncReloader.func_219560_a(SourceFile:70) ~[?:?]
at java.util.concurrent.CompletableFuture$UniCompletion.claim(CompletableFuture.java:543) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:667) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:646) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:488) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.postFire(CompletableFuture.java:575) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:649) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:456) ~[?:1.8.0_282]
at net.minecraft.resources.AsyncReloader.func_219557_a(SourceFile:71) ~[?:?]
at net.minecraft.resources.AsyncReloader.func_219560_a(SourceFile:70) ~[?:?]
at java.util.concurrent.CompletableFuture$UniCompletion.claim(CompletableFuture.java:543) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:667) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:646) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:488) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.postFire(CompletableFuture.java:575) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:649) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:456) ~[?:1.8.0_282]
at net.minecraft.resources.AsyncReloader.func_219557_a(SourceFile:71) ~[?:?]
at net.minecraft.resources.AsyncReloader.func_219560_a(SourceFile:70) ~[?:?]
at java.util.concurrent.CompletableFuture$UniCompletion.claim(CompletableFuture.java:543) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:667) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:646) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:488) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.postFire(CompletableFuture.java:575) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:649) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:456) ~[?:1.8.0_282]
at net.minecraft.resources.AsyncReloader.func_219557_a(SourceFile:71) ~[?:?]
at net.minecraft.resources.AsyncReloader.func_219560_a(SourceFile:70) ~[?:?]
at java.util.concurrent.CompletableFuture$UniCompletion.claim(CompletableFuture.java:543) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:667) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:646) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:488) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.postFire(CompletableFuture.java:575) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:649) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:456) ~[?:1.8.0_282]
at net.minecraft.resources.AsyncReloader.func_219557_a(SourceFile:71) ~[?:?]
at net.minecraft.resources.AsyncReloader.func_219560_a(SourceFile:70) ~[?:?]
at java.util.concurrent.CompletableFuture$UniCompletion.claim(CompletableFuture.java:543) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:667) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:646) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:488) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.postFire(CompletableFuture.java:575) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:649) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:456) ~[?:1.8.0_282]
at net.minecraft.resources.AsyncReloader.func_219557_a(SourceFile:71) ~[?:?]
at net.minecraft.resources.AsyncReloader.func_219560_a(SourceFile:70) ~[?:?]
at java.util.concurrent.CompletableFuture$UniCompletion.claim(CompletableFuture.java:543) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:667) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:646) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:488) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.postFire(CompletableFuture.java:575) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:649) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:456) ~[?:1.8.0_282]
at net.minecraft.resources.AsyncReloader.func_219557_a(SourceFile:71) ~[?:?]
at net.minecraft.resources.AsyncReloader.func_219560_a(SourceFile:70) ~[?:?]
at java.util.concurrent.CompletableFuture$UniCompletion.claim(CompletableFuture.java:543) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:667) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:646) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:488) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1609) ~[?:1.8.0_282]
at java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1596) ~[?:1.8.0_282]
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289) ~[?:1.8.0_282]
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056) ~[?:1.8.0_282]
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692) ~[?:1.8.0_282]
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:175) ~[?:1.8.0_282]

Please say if thats enough - its my first post like this

Yes, I see the issue now!

Since I added a blacklist to the egg magic, Minecraft is now complaining that is can not find the blacklist in your json file. You can look at the egg magic json file here. The issue should be fixed if you add the blacklist attribute to your json file.

Thank you!
Just added those lane to datapack and everything works now!
We can close that one ^^

Great! I am glad I could help.