i was having the same issue. followed some of the above posts about removing basemaps. Now i am unable to remove the offline webmap (collector crashes when i attempt to remove it) or open it since it has no basemap associated.
Also noticed that seems like there are some other major glitches with creating maps to take offline. I could not create a new one (the offline enable option is not there), i was able to copy and change one of the maps that i had already been using offline though.
Very frustrating since i use the offline data collection daily and am relying on it to collect data. We need a fix ASAP!