A student of mine (running a windows 8 laptop) developed a problem when they added an item to a group that included this URL:
http://www.johncoulthart.com/feuilleton/2010/05/11/frank-frazetta-1928%e2%80%932010/
The error message indicated that zotero could not store the file because of a number of possible reasons, including the filename, and there was an encoding artifact in the error message where the 'hyphen' would be (UTF-8 en dash %e2%80%93).
While all the entries appeared to be in the group, any attempt to open the entry would result in the error that the item could not be located.

I deleted the item from the group and then tried to sync, and the error persisted.

I manually deleted the entire local zotero directory and repopulated, and the error reappeared.

It seems at present zotero is hung trying to store something that should no longer be stored. I don't know how to cure the encoding issues, I don't know how to solve the sync issue, and I don't know whether the inability to locate anything will resolve itself once the problem with the one file is resolved.

I did not bother to try and fix the db because I assumed that recreating from scratch from the source would address this.

What is my next step? This is a most unfortunate time for this to crop up.