Worshipper – “Betrayal” [2nd campaign, 1st mission]

try enabling autosave in the options and play through it again. When the cutscene where Nikolai is killed appears, the game will save. If the same problem with the second campaign being inaccessible occurs, load the latest autosave. This worked for me, and I didn't install any patches.

Added 1 hour ago
By the way, where is the red keymaster, and where should I go?