I am sorry for the inconvenience caused to you.
Please follow the steps below and see if this helps.
1. File>Plug-In Manager...
2. Select "Flickr" on the left.
3. On the right, expand the "Plug-in Author Tools" section, and click "Reload Plug-in".
Also, have you tried contacting flickr?
This happens to me as well from time to time. It is almost certainly a Flickr issue.
Thanks! I tried, but it didn't help. I haven't contacted Flickr yet, because I guess I am finished long before they will fix this anyway.
I also have almost the same problem with Flickr's own new Uploadr.
As dj_paige observed, this is definitely a problem with the Flickr service that comes and goes. It seems that since the big "upgrade" to Flickr a year or two ago, and with Yahoo's corporate troubles, Flickr's reliability has been getting noticeably worse.