Displacement is hanging around and you can get stuck in it on the ceiling: -5440 -6144 -44
Had some clients crashing so I went and diagnosed that (I saw earlier in the thread you were trying to fix the problem as well).
You have some (5 total) func_illusionary which are grouped together over a very large area, if you want to do this you need to make sure to set disableshadows 1 (and potentially disablereceiveshadows 1, I do anyway) on those groups. They cast dynamic shadows so when they come into view for players, the lighting engine freaks out when it has to allocate a bunch of memory (based on the bounding box size of the grouped prop) creating all these dynamic shadows. Any grouped objects (be it props or brush entities) which bounding box is over a few thousand units that cast dynamic shadows will cause problems (CUtlLinkedList overflow), especially for people who aren't running an FPS config.
Ive been running a fix where I force set those func_illusionary to disable shadows, and it seems its fixed the issue.
Some feedback as well, 2nd stage last point, the vast majority of people never knew that there is an underwater flank, its just not visible enough and the server has played the map multiple times.