Forge: 1.15.2-31.2.9
citadel: citadel-1.1.9

logs:
latest.log

Having a similar issue using the new version of Citadel. Local world hangs when preparing spawn area. The common thread seems to be this error:

[Server-Worker-18/ERROR]: Caught exception in thread Thread[Server-Worker-18,5,SERVER] java.lang.RuntimeException: Provider com.github.alexthe666.citadel.server.CitadelServerEvents$1.getCapability() returned null; return LazyOptional.empty() instead!

The error goes on to mention the Mutant Beasts mod in my case, and Curios in the log @SDUBZ posted. Reverting Citadel back to 1.1.7 resolved the issue for me.

same in singeplayer it just wont load at all
citadel-1.1.10
Forge 31.2.0

I'm also having this issue.
Forge: 1.15.2-31.2.9
citadel: citadel-1.1.9

I'm updating to Forge: 1.15.2-31.2.19 at the moment.
reverted Citadel back to 1.1.7 until this is resolved.

Also having this issue.
Citadel-1.1.10
Forge 31.2.9

Reverting to 1.1.7 fixed my issue.

still happens with citadel-1.1.10

Happens with citadel-1.1.10 for me too

• Minecraft Version: 1.15.2
• Operating System: Linux (amd64) version 4.15.0-99-generic
• Java Version: 1.8.0_251, Oracle Corporation
• Forge: 31.2.20
• citadel-1.1.10
  crash-2020-06-16_15.14.58-server.txt
  crash-2020-06-16_15.15.14-client.txt

Reverting to 1.1.7 also fixed my issue

Fixed in version 1.1.11.