Moved this issue report into a new discussion to keep it organized.
It appears that that after saving and quitting and then in my case reopening the save a day later, I as the host can still join the save and use the mod with full functionality, but my friend who was able to play yesterday keeps getting a desync when trying to join. There was no way for other players to join afterwards as they all kept getting a desync.
More interesting is that even previous saves do no longer work in multiplayer that case. Players other than the host just keep getting a desync.
In the log:
40.222 Info ClientMultiplayerManager.cpp:608: UpdateTick(2909583) changing state from(ConnectedLoadingMap) to(TryingToCatchUp)
40.392 Error GameActionHandler.cpp:2808: Multiplayer desynchronisation: crc test (heuristic) failed for crcTick(2909591) serverCRC(3834096706) localCRC(2773842740)
40.393 Info ClientMultiplayerManager.cpp:608: UpdateTick(2909592) changing state from(TryingToCatchUp) to(DesyncedWaitingForMap)
40.457 Info ClientMultiplayerManager.cpp:741: updateTick(2909593) received player (1) desynced
41.220 Info ClientMultiplayerManager.cpp:747: Received mapReadyForDownload
41.220 Info ClientMultiplayerManager.cpp:608: UpdateTick(2909593) changing state from(DesyncedWaitingForMap) to(DesyncedCatchingUpWithMapReadyForDownload)
Then followed by hundreds of these lines:
41.220 Error GameActionHandler.cpp:2808: Multiplayer desynchronisation: crc test (heuristic) failed for crcTick(2909592) serverCRC(1133711296) localCRC(390684994)
and ending with
41.746 Info ClientMultiplayerManager.cpp:1255: Transferring ownership of scenario to ParallelScenarioSaver (global = true).
41.747 Info ClientMultiplayerManager.cpp:608: UpdateTick(18446744073709551615) changing state from(DesyncedCatchingUpWithMapReadyForDownload) to(DesyncedSavingLocalVariantOfMap)
45.487 Info ParallelScenarioSaver.cpp:49: Deleting active scenario (global = false).
45.608 Downloading file /Users/username/Library/Application Support/factorio/temp/mp-download.zip (27223926 B, auxiliary 118 B, 54125 blocks)
45.612 Info ClientMultiplayerManager.cpp:608: UpdateTick(18446744073709551615) changing state from(DesyncedSavingLocalVariantOfMap) to(DesyncedDownloadingMap)
59.085 Finished download (13 s, 27.2 MB, 2.0 MB/s)
59.121 Info ClientMultiplayerManager.cpp:727: updateTick(18446744073709551615) map download finished creating scenario
59.121 Info ClientMultiplayerManager.cpp:608: UpdateTick(18446744073709551615) changing state from(DesyncedDownloadingMap) to(DesyncedCreatingReport)
