I am using Cubase 9.030.
Sometimes (quite often), a project sort of “loses it” and suddenly undo doesn’t work, I can view history, which does not contain record events, but I can’t undo using the history function. In the same instances, the recently recorded parts are not possible to select or do anything with, like going into the editors. Restarting Cubase fixes it - temporarily. But it is a major pain in the backside. It’s been there for several Cubase 9 updates, not new with the latest one.
System:
Win10 64 bit latest update.
Steinberg UR28M sound card
Tons of plugs, but current project uses only
S-Gear 2 (a guitar plug)
East-West play (with one ProDrummer loaded).
Revalation
Reverence