Reason: Game use too much RAM
Way to solve it: Disable or unsub the mods with high texture quality and use the graphic setting below...
Anti-Aliasing Mode - 2xMSAA
Filtering Mode - Anisotropic 2x
Vsync - Disable
Shader Detail - High
Effect Detail - Low
Model/Texture Detail - Low
Multicore Rendering - Enable
Paged Pool Memory Available - High
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Reason: Too many zombies (maybe), prop_dynamics (maybe), scripts (confirmed), Author hate other people's scripts (confirmed)
Way to solve it:
1. Open Steam > Right click 'left 4 dead 2' > Press 'set launch options' > Enter '-num_edicts 2048' and press 'ok'.
2. Open Left 4 Dead 2 > On main menu, press 'add ons' > Change workshop tags to 'Scripts' > Deselect all mods in the list (except 'Chernobyl: Part 1') > Press 'done' to apply
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Now the campaign should work fine...