Halflife 2 Mod Dark Interval quits right at startup, the solution.
I recently installed the free mod Dark Interval. Unfortunately, it crashed right at startup. It appeared briefly the start window, but ended directly after again.
The solution in this case was to set a few settings in the “Source SDK Base 2013 Singleplayer”. Right click, then on “Properties” and in the settings under “Beta” set the value to “upcoming”.
After that the mod ran without problems.