Security token needs to be added to the "forumdisplay_changethreadsowner" template to make this work with 3.6.10.
Load Style Manager locate the template under "Forum Display Templates"
Under:
HTML Code:
<input type="hidden" name="s" value="$session[sessionhash]" />
Insert:
HTML Code:
<input type="hidden" name="securitytoken" value="$bbuserinfo[securitytoken]" />
Save. Refresh the Change Threads Owner form (so it picks up the new template version) and it works fine. I just fixed mine too because I forgot to test it before upgrading to 3.6.10.
If you do not add the security token, it won't work.