Problem:
You can't create a file on a Windows pc like COM8/COM7/....* and BloodMagic try's to. My ingame name is COM8. So there is a problem. :(

Possible solution:
Don't use ingame names for this it would be better to use the UUID or something else .
So everybody can play again without problems. :D

Informations:
Minecraft: 1.7.10
Forge: forge-10.13.4.1492
BloodMagic: 1.7.10-1.3.3-13

Possible error-Log:
[23:29:07] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]: java.io.FileNotFoundException: D:\Fabian\Documents\Curse\Minecraft\Instances\test\saves\New World\data\COM8.dat (Das System kann die angegebene Datei nicht finden)
[23:29:07] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]: at java.io.FileOutputStream.open(Native Method)
[23:29:07] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]: at java.io.FileOutputStream.(FileOutputStream.java:213)
[23:29:07] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]: at java.io.FileOutputStream.(FileOutputStream.java:162)
[23:29:07] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]: at net.minecraft.world.storage.MapStorage.func_75747_a(SourceFile:92)
[23:29:07] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]: at net.minecraft.world.storage.MapStorage.func_75744_a(SourceFile:75)
[23:29:07] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]: at net.minecraft.world.WorldServer.func_73042_a(WorldServer.java:838)
[23:29:07] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]: at net.minecraft.world.WorldServer.func_73044_a(WorldServer.java:802)
[23:29:07] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]: at net.minecraft.server.MinecraftServer.func_71267_a(MinecraftServer.java:318)
[23:29:07] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]: at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:106)
[23:29:07] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]: at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:427)
[23:29:07] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]: at net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:685)

When does this occur? And I HIGHLY doubt it is because I am using your name, pretty sure that's the normal player.dat that MC creates for your character.

Yup, that's the data file that Minecraft creates. If it were because of your name, you'd never be able to even create a world.

I tried to make a file in Windows named COM8.dat. It stops you. http://i.imgur.com/NtlAhoO.png

@COM8 does Baubles or Thaumcraft research save for you? They also save via username, so we should see a similar problem. The player.dat files use UUIDs not usernames for naming so it's not an issue.

And again, this is a file generated by Minecraft. It has nothing to do with BloodMagic, really.

How? The file is being created in the SoulNetworkhandler with the owner's name. https://github.com/WayofTime/BloodMagic/blob/master/src/main/java/WayofTime/alchemicalWizardry/api/soulNetwork/SoulNetworkHandler.java

I think the main issue here is that this is a once-in-a-blue-moon occurance. Eventually I will change to using UUIDs, but in all seriousness this is something that I doubt anyone could have seen coming.

(That file is the saved data, yes)

Agreed, I didn't even know COM8 and such were reserved file names.

@Nincodedo Yes Thaumcraft / Baubles have the same problem and I already reported it.
https://github.com/Azanor/thaumcraft/issues/1715
LogisticsPipes has the same problem.
RS485/LogisticsPipes#775

I'm going to have to close this right now. I can assume that because you decided on the name COM8, you knew what you might get in to if a mod creates a file with your name. Therefore I am going to close this issue and EVENTUALLY rework this.