This is to my knowledge due to the coruption of the col.50 file (the .session file in 3.7). Deleting this file, creating a new session and dragging the old folders over into the new session is the workaround. I would avoid using albums as they are basically aliases; using folders means you have these files in the finder independent of C1. Albums are stored in the col.50 file so if it corrupts you lose the edit (not the actual raws though). The latest version (5.1.1) seems much more stable, i'd recomend upgrading