Same here, also on 3440x1440 (1080 Ti, latest driver, fresh W10 install).
Pattern slightly different: I can not join public games without ctd’ing within one or two minutes after join. No matter what I do or where I go.
So I create my own games. Works normally for some time - then randomly ctds without knowing why. Eg entered lvl 3 for Mephisto - came through the door, and freeze (had to restart the pc, not even alt tab etc worked). Maybe was on the map for two seconds.
Tech specs all met, no gpu overheating or other possible hardware issues. Graphics on medium preset.
My guess: It’s something about the 21:9 resolution. Possibly together with the 1080 Ti - but not sure, am not a tech expert. Most likely we have to check/uncheck one or two boxes only. Oh well.