This is for anyone in the future that runs into this error like I just did and broke down crying because they lost hundreds of hours only for the map to corrupt.
The fix is to go to map_source > zm > zm_yourmapname.map
And replace it with radiant > autosave_backup > zm_yourmapname_with_a_recent_date.map
Make sure you rename the new file to match the original.
Sorry for the necro, but this thread is the first result when you search this error on google and nobody on the internet had a solution. i hope this helps people.