For some reason, everyone turns invisible suddenly, and some props like the fountain, trees doors all dissapear. They would need to relog to see everything and everyone. I need a fix ASAP

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This has been an issue even since Garry’s Mod 12, if I recall correctly. Doing various google searches only results in similar no-answer threads. Just as you stated, the only way I know to fix the issue is to rejoin the server.

If anyone has valuable information on what causes or can fix this glitch, it would be greatly appreciated.

This has happened before on my server and usually seems to be caused by having too many emitters.

try minimizing your Garry’s Mod then going back to it or typing mat_refresh_textures/materials… if none of those work just restart your garrysmod