I just had a weirdest bug, which I don't know how to replicate, but it really freaked me out. I was working on a map, saved it, then started Quick Play.
I got the error "level_01" doesn't exist which was odd because I was staring at the map in the editor (I've been working on level_01 for a week).
I thought I should try reopening the map so I closed it.. and it completely disappeared from the list of maps inside RIAB and also from the list of recently opened maps.
I closed RIAB down and restarted it.. and thankfully the map came back, however, it was pretty scary. I took a backup of my project just in case.
Sharing this in case anyone else has experienced the same bug.