After starting a new game with the HEAVILY MODDED Modpack it went fine for ~200 hours before we've suddenly started getting desyncs, first sometimes but after a while right away when one person was joining the hoster (switching host also didn't work).
We've been trying to figure out which mod might be the cause but we haven't been able to get any closer to a solution. At this point we've disabled a ton of mods that we weren't using in the hopes it would help (it did, but only a little and only sometimes).
Even when we thought it might be LTN, so we disabling the entire mod but no luck.
So right now we're just playing in turns
I first posted on the HMM page itself but it's been over 2 weeks with no response.
I've uploaded the latest desync report to Dropbox, it can be found here.
118.346 Script @_heroturrets_/control.lua:470: control.local_on_start
So I'll be checking with my brother as soon as possible if it's perhaps this mod that's causing the issues. (I'll update this part once I know more)
If you need any info, please let me know and I'll try to help as much as possible!