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XOOPS 2.0.15 has 1 confirmed bug which I think is causing some issues - an insertLine function from the form.php file, if I remember correctly (read it somewhere on this site, a search should find it)
This activation key issue seems to be common, but it's not confirmed by the core devs yet - at least, I haven't seen any posts by them about it so far. It's great that the people posting in this thread are alerting the core devs to the problem by using the bug tracker, hopefully we'll get some info about it soon.
For the problems with some modules and 2.0.15 I believe that is because those modules are bypassing the XOOPS API and directly manipulating core database tables. That's something you should take up with the module developers, nothing the core devs can do about that.
This is all info I've picked up by reading posts on this site, and following the core dev forum postings and SVN commits. For me XOOPS 2.0.15 works perfectly