Hello,
I discovered this problem with MalisisDoors.
Maybe is good to mention, that the server is running on SpongeForge and on BungeeCord network (maybe it is an important information?)

Crash report: https://pastebin.com/Xtym3E1v

MalisisCore: 6.1.7
MalisisDoors: 7.1.1
Forge: 14.23.0.2512

EDIT:// Found out, that the problem was actually in combination SpongeForge and BungeeCord. Sorry for this issue.