My install is Drupal 6.3 + FCKeditor 2.6.3 and 6.x 1.3 beta2 + IMCE
(I also tried 2.6.1 and 2.6.2 but it did not resolve the problem)
I am trying to add a thumbnail to my node that links to a bigger picture. I use IMCE to browse the server, select the thumbnail, and send the URL to FCKeditor. That part works.
Then I click "Link", choose the bigger picture, "send to FCKeditor", and nothing. It simply refuses to transfer the URL.
Both my 6.3 sites exhibit this problem. 5.x works fine.