After I upgraded to 3.5 I have had this error whenever I try to publish to my Flickr account (pro)
Flickr API returned an error message (function upload, message Invalid API Key (Key has invalid format))
The Freidl plugin works fine but the publish feature does this error each time.
HACK: In a pinch you could try renaming Flickr.lrplugin and copying the Flickr.lrplugin from 3.4 into the same directory. (Assuming that there is something wrong with the current plug-in -- I haven't noticed if there is, but I rarely use Flickr.)