I updated Source.Python by extracting the latest build into my server folder. This seems to have downgraded the data to an older version than the latest automatically downloaded source-python-data.zip.
It crashes whenever the first player connects to the server.
I know @NosferatuJoe ran into a similar issue, but after using the sp update command. I suspect this is for the same reason. I can also confirm that the issue happens in this case.
Deleting csgo\addons\source-python\data\source-python-data.zip fixes the issue.
I suspect extracting the latest build again after running the server at least once will also cause the issue.
#409 seems to be the same underlying problem.