There does appear to be an additional problem that causes this error - it may have been introduced in one of the recent releases (since I haven't previously seen this particular problem).

I have no idea what triggers the problem. But effectively, any new search added to the download queue cannot be moved to the library. Shutting down LimeWire and restarting it appears to cure the problem.

I shall be reporting this as a new bug - could those who have seen this problem try to take as much note of the circumstances as possible and comment here.

Mark