After many hours of debugging i managed to trace a java.lang.IllegalStateException: TickNextTick list out of synch server crash back to this mod's MutableBlockPos and (probably) dynamic trees (and possibly random patches?).

This Exception is thrown when pendingTickListEntriesTreeSet and pendingTickListEntriesHashSet differ. This seems to be caused by NextTickListEntrys not being removed from the HashSet if their .position is an instance of MutableBlockPos. I don't know why this happens, it might be related to an issue with equals and compareTo as described here.

Vanilla also has a MutableBlockPos class, with one difference: Vanilla's BlockPos provides a method toImmutable(), which for the normal BlockPos is implemented as a simple return this;. In the vanilla MutableBlockPos though this method is overriden with return new BlockPos(this);. Because NextTickListEntry.<init> calls toImmutable, all positions should be normal BlockPos, but because thut.api.maths.Vector3$MutableBlockPos doesn't override toImmutable, it uses the identity inherited from BlockPos instead.

I managed to fix this for me by adding a check in pendingTickListEntriesHashSet#add and, if the position is not an instance of BlockPos, replacing it using reflection.

Since i am on 1.12.2 and this repo only has the source for more recent versions i can't build a fix, but as vanilla minecraft now provides a MutableBlockPos it should be relatively simple to remove ThutCore's MutableBlockPos (or at least override toImmutable).

this is fixed for the next 1.15.2 update, that was some really old stuff that I forgot to update, from before vanilla had an easily accessible immutable block pos...

you can find the obsolete 1.12.2 code here if you did want to make the change on your end: https://github.com/Thutmose/Thut/tree/1.12.x

I forgot that saying "fix" would automatically close the issue...

this is fixed in the new 1.15.2 release