okay, I'm just looking at the metadata.calibre file in a text editor and I think I may have found the problem.

one of your book entries has a "thumbnail" entry. none of mine had this entry and I didn't know what format it would take. it is actually a number, followed by another number followed by a string, so that is probably why BIbliotheca is failing.

I'll try to have a look tonight at home and fix it.

andy.