Happened mid-session during an autosave.

2020-10-27 09:21:06.824|Main Thread>    Exception occurred: System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary`2.ValueCollection.Enumerator.MoveNext()
   at System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
   at System.Linq.Enumerable.<CastIterator>d__97`1.MoveNext()
   at Sandbox.Game.Entities.MyVoxelMaps.GetVoxelMapsArray(Boolean includeChanged)
   at Sandbox.Game.World.MySandboxSessionDelta.Prepare(MySession session, String saveName)
   at Sandbox.Game.World.MySession.GetSnapshot(String newSaveName)

All following autosaves failed accordingly:

2020-10-27 02:31:06.261|Main Thread> Auto-Save: Started
2020-10-27 02:31:06.261|Main Thread> Auto-Save: Finished

The issue was unnoticed until a player informed me about the lost progress. After a server restart, it rolled back 6 hours:

2020-10-27 03:00:47.925|Main Thread>    Loading session: C:\ProgramData\MedievalEngineersDedicated\ME-Server-1\Saves\Land-of-Seduction
2020-10-27 03:00:47.925|Main Thread>    Save version: 0.7.2.9224146
2020-10-27 03:00:47.943|Main Thread>    Found save lock file, restoring last backup before continuing load.
2020-10-27 03:00:47.967|Main Thread>Backup> Info: Restoring backup 10/27/2020 09:00:24 into C:\ProgramData\MedievalEngineersDedicated\ME-Server-1\Saves\Land-of-Seduction.
2020-10-27 03:00:49.920|Main Thread>Backup> Info: Deleting backup data.