Prerequisite: We validated game files. All vanilla.
My friends and I encountered this bug first time this month, during R4C3. And again in R4D2.
Possible reproducability: Uncovering big parts of the mission, so they are visible on the TAB-Map and frequently watching the TAB-Map. Happened as host and as client.
Description of the error, always leading to game crashes:
The first time in R4C3 we needed around 2 - 3 hours, until the host first encountered audio skips. Missing sound for shoting. Muted enemies. Until his game became completely mute. Using the TAB-Map at this point started lowering his FPS. Occasionally freezing it for 1 second. Then running without problem for 2 seconds. This situation was permanent and became worse, the more often the map was opened from this point onward. So worse that the freeze time became 15 seconds and 2 seconds running. Until the game stopped responding entirely. Also checking for player-boosters caused that to happen.
As a client it was "fun" at first, because now it was quite easy to reduce the enemy count.
This happened in R4C3 and R4D2.
Additionally in R4D2 the host had one scout, that was about to alert his own horde. During his cry however, suddenly the "cry"-sound started looping into oblivion, for the host only. It became so bad, that he had to mute the game entirely. But at first no other problems like FPS drops happened. Until we saw the enemy "teleporting" towards us. They stood still for around 10 seconds and then suddenly moved for about one. We had a bot during D2 and his behaviour was about the same. The worst was however that mission-objectives also froze up. Having reached the end after doing everything else, we thought that it would fix itself during the endscreen, but having finished the extraction-scan... nothing happened. For 30mins we waited. Enemies and Bots still moving from time to time. We taking damage from the gas. But no "mission finished".