I have removed all mods (and config folder) except of the fabric api and the custom portals mod. Tested it with and without the completeconfig mod.

The relevant part seems to be:
Caused by: java.lang.NoSuchFieldError: checkbox at me.lortseam.completeconfig.data.BooleanEntry.<init>(BooleanEntry.java:26) ~[completeconfig-base-2.1.0-ac48f735a60effdb.jar:?]

Here is the complete crash output:
[01:29:41] [main/ERROR]: Failed to start the minecraft server java.lang.RuntimeException: Could not execute entrypoint stage 'main' due to errors, provided by 'customportals'! at net.fabricmc.loader.impl.FabricLoaderImpl.lambda$invokeEntrypoints$2(FabricLoaderImpl.java:388) ~[fabric-loader-0.15.6.jar:?] at net.fabricmc.loader.impl.util.ExceptionUtil.gatherExceptions(ExceptionUtil.java:33) ~[fabric-loader-0.15.6.jar:?] at net.fabricmc.loader.impl.FabricLoaderImpl.invokeEntrypoints(FabricLoaderImpl.java:386) ~[fabric-loader-0.15.6.jar:?] at net.fabricmc.loader.impl.game.minecraft.Hooks.startServer(Hooks.java:63) ~[fabric-loader-0.15.6.jar:?] at net.minecraft.server.Main.main(Main.java:101) [server-intermediary.jar:?] at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:470) [fabric-loader-0.15.6.jar:?] at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:74) [fabric-loader-0.15.6.jar:?] at net.fabricmc.loader.impl.launch.knot.KnotServer.main(KnotServer.java:23) [fabric-loader-0.15.6.jar:?] at net.fabricmc.loader.impl.launch.server.FabricServerLauncher.main(FabricServerLauncher.java:69) [fabric-loader-0.15.6.jar:?] Caused by: java.lang.NoSuchFieldError: checkbox at me.lortseam.completeconfig.data.BooleanEntry.<init>(BooleanEntry.java:26) ~[completeconfig-base-2.1.0-ac48f735a60effdb.jar:?] at me.lortseam.completeconfig.data.Entry.create(Entry.java:42) ~[completeconfig-base-2.1.0-ac48f735a60effdb.jar:?] at me.lortseam.completeconfig.data.EntrySet.lambda$resolve$1(EntrySet.java:31) ~[completeconfig-base-2.1.0-ac48f735a60effdb.jar:?] at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197) ~[?:?] at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:179) ~[?:?] at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:992) ~[?:?] at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509) ~[?:?] at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499) ~[?:?] at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:150) ~[?:?] at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:173) ~[?:?] at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[?:?] at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:596) ~[?:?] at me.lortseam.completeconfig.data.EntrySet.resolve(EntrySet.java:32) ~[completeconfig-base-2.1.0-ac48f735a60effdb.jar:?] at me.lortseam.completeconfig.data.Parent.resolveContainer(Parent.java:58) ~[completeconfig-base-2.1.0-ac48f735a60effdb.jar:?] at me.lortseam.completeconfig.data.ClusterSet.resolve(ClusterSet.java:13) ~[completeconfig-base-2.1.0-ac48f735a60effdb.jar:?] at me.lortseam.completeconfig.data.Parent.resolve(Parent.java:108) ~[completeconfig-base-2.1.0-ac48f735a60effdb.jar:?] at me.lortseam.completeconfig.data.Parent.lambda$resolveContainer$6(Parent.java:97) ~[completeconfig-base-2.1.0-ac48f735a60effdb.jar:?] at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:183) ~[?:?] at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197) ~[?:?] at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:179) ~[?:?] at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:992) ~[?:?] at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509) ~[?:?] at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499) ~[?:?] at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:150) ~[?:?] at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:173) ~[?:?] at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[?:?] at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:596) ~[?:?] at me.lortseam.completeconfig.data.Parent.resolveContainer(Parent.java:97) ~[completeconfig-base-2.1.0-ac48f735a60effdb.jar:?] at me.lortseam.completeconfig.data.Parent.resolve(Parent.java:110) ~[completeconfig-base-2.1.0-ac48f735a60effdb.jar:?] at me.lortseam.completeconfig.data.Config.lambda$new$0(Config.java:51) ~[completeconfig-base-2.1.0-ac48f735a60effdb.jar:?] at me.lortseam.completeconfig.data.Config.deserialize(Config.java:94) ~[completeconfig-base-2.1.0-ac48f735a60effdb.jar:?] at me.lortseam.completeconfig.data.Config.load(Config.java:121) ~[completeconfig-base-2.1.0-ac48f735a60effdb.jar:?] at dev.custom.portals.CustomPortals.onInitialize(CustomPortals.java:36) ~[custom-portals-3.2.0-1.19.2.jar:?] at net.fabricmc.loader.impl.FabricLoaderImpl.invokeEntrypoints(FabricLoaderImpl.java:384) ~[fabric-loader-0.15.6.jar:?] ... 6 more

Edit:
Okay so I found out it works with the loader fabric-loader-0.14.25
but im using the loader with fabric-loader-0.15.6. the server has already +200 mods. Downgrading the loader creates a lot of incompatibilities.
Could you please fix it for 0.15.6?

It's deprecated because it's not being updated to 1.21, so I do need to switch to YACL anyway at some point. I'll reopen this to remind myself to do that

Happens on 1.20.1 as well, only on the server side, however. No viable version of CompleteConfig has been found.

@praecipitator I know this is an almost a two month old comment by now, but the version of completeconfig for 1.20.4 is also compatible with 1.20.1 despite what it says, and should fix the server issue

I see CompleteConfig is depreacted. Would using YetAnotherConfigLib be an option?

Known CompleteConfig issue. Was fixed in v2.5.2 of CompleteConfig, but this is for 1.20. They would need to fix it on their end for 1.19 as well. Otherwise I'm afraid the only option is to downgrade the loader.

@Palyon-dev So what I'm getting from the 1.20.1 comment is that I use the 1.20.4 version of the mod in my 1.20.1 server. Even when I use that, it requests Minecraft version 1.20.4 and not 1.20.1 so not sure what you mean by the fix.

Disregard that last comment. I am a complete idiot. What you do is install CompleteConfig 2.5.2 and then install version 3.3.1 for 1.20.1

CompleteConfig has been replaced with YACL for versions for Minecraft 1.20.1 and up. Unfortunately switching to YACL for the 1.19.2 release would be a major undertaking as the 1.19.2 version of YACL does not have many of its modern features that my mod now relies on. I'll once again have to leave it up to the developer of CompleteConfig to port their fix for this issue to 1.19.2 if they choose.