I also have this problem in Ubuntu Linux 10.04, and it occurs in Kompozer 0.8b1, 0.8b2, and 0.8b3. To be more specific, when I copy html source from a template I have in a text file and then try to paste it into the "source" view, the paste fails and is grayed out. This happens only when I open Kompozer and then open an existing html file that I'm trying to edit. After opening the html file, the only click I do on the interface is to select the "source" view. Then I try to paste the selected text from the external file into the source in the Kompozer "source" window. Paste fails in this instance.

There is a work-around I found that might indicate the source of the problem. If I click in the work area of the "design" view one time to cause the cursor to appear there, then I can copy and paste into the "source" view and also into the "design view" without fail, and it will continue to work throughout that session until the application is closed.