This issue only occurs when more than one player is on the server (except SourceTV when not recording a demo?).
Synergy does not have this issue, and I recall getting this same error on Source 2013 MP.
When you do get sent to the main menu, the server is still running, and other people are able to play perfectly fine, but you are stuck on the main menu.
When you run changelevel2/hit a level transition, and SourceTV records a demo, you get put on the main menu with the console flooding with this error:
This error is located in engine/sv_ents_write.cpp https://github.com/quiverteam/Engine/blob/upcoming/src/engine/sv_ents_write.cpp#L627
This issue only occurs when more than one player is on the server (except SourceTV when not recording a demo?). Synergy does not have this issue, and I recall getting this same error on Source 2013 MP.
When you do get sent to the main menu, the server is still running, and other people are able to play perfectly fine, but you are stuck on the main menu.
If you want to cause this issue easily, clone this with recursive and build: https://github.com/Demez/demez_hl2
Enter these commands into the console:
And start any map (like d2_coast_01), and cross a level transition.