Make sure you check following things before you open a new issue:

[X ] Check that you use Java 8
[X ] Check that you use the latest MinecraftForge version available for your version of Minecraft
[X ] Check that you use the Minecraft version supported by the mod

Put the answers below the following questions:
-What version of Minecraft do you use?
1.10.2

-What version of MinecraftForge do you use?
forge-1.10.2-12.18.3.2185

-What version of SanLib do you use?
1.10.2-1.0.1

-What is the issue / bug you have encountered? Give us a short description.
The mod works perfectly fine while spawning clay soldiers in the singeplayer. If I set up my Minecraft-Server with the Mod every player get next to a spawned clay soldier get disconnected because of an FatalError - no further clues why or crashreports :-/ The server it self is still running, but you have to rollback the world to a state before spawning, because the crashed players can't login.

-How can it be reproduced?
I can reproduce this error while setting up a new minecraft forge server with the clay soldier mod and the sanlib and with my gamed.de-Server

Please post your log file (either in /logs/fml-client-latest.log or /logs/latest.log)
Latest FML Server Log: https://gist.github.com/Accurexx/349510e3086d20b26cc26a1691bc26cc
Latest Server Log: https://gist.github.com/Accurexx/efccaa968c137badf8c9b50690363037

Can you give me the log from a client that is crashing? The server logs don't say anything except the disconnections happening.

hm... could you try with just Clay Soldiers and SanLib installed and see if that works?

In about 3 hours. I'll give you an update as soon as possible.

Yes, of course. I though the client-log might be useless, since the mod works fine in singleplayer, so it must have to do something with the server :D

I hope this is the right file, at least the time matches.

https://gist.github.com/Accurexx/9346c76526b8d54f119fd3f6123c5d47

There are 2 regions within the log where thinks go weird (at least when i open the log with the notepad).
If it's the right file, the first one should be the clay soldier placement, the second one the reconnection.

The FatalError still occurs.

I've deleted all the mods from the server, except "ClaySoldiersMod-1.10.2-3.0.0-alpha.1" and "SanLib-1.10.2-1.0.1", same for the client.

FML Server log: https://gist.github.com/Accurexx/c5a32a6c1e839b8046c5769405f5871e
Client log: https://gist.github.com/Accurexx/eb447307efcb67cdb8b23a34c45f4983

I found the problem, I need to change the way my custom data serializer is initialized.

Ok, great. Were you able to reproduce the bug? If not, let me know if you need some tests.

I've released a new update. Please tell me if I indeed fixed this problem. Thanks :)

There are still the "Unknown recipe warnings" within the logs, idk if they should be there ;-) But the mod seems to work. Thanks a lot!

yea, these errors are due to me not adding them properly to the registry, I'll have to fix that.