I've just installed the "Feed Handler" and "Feed Tools" addons to my 2.2.0 RSSOwl on 64bit Win 7.
On startup I now get a couple of error boxes labelled "NSIS ERROR" with the message "Error writing temporary file. Make sure your temp folder is valid." What's this about and what do I need to do to resolve the problem? Disabling the addons removes the problem, but the features sounded as though they were worth investigating!