I don't know what to say really.. if it happened on and off it looks like more a Server issue than Music Collection's...
I mean, music collection acts always the same way, logically.. so maybe it was a temporary issue on the server..
please check again. the problem is the onw I said, for some reason the folder /cache/muscol/artists did not exist.
what I don't understand is that I saw it working, so at some point it existed. did you erase it yourself?