I get the same results as you when I switch themes - tinymce works normally - and I also get errors relating to an old block when I switch debugging on. The problem is I'm using a custom theme and I don't want to have to change it (unless I really have to), and the old block is one I use in almost every course on my site. It's the 'Course Results' block and it hasn't been updated since 2.2.
Here are the debug errors I get:
The block course_results does not define the standard capability block/course_results:addinstance
line 606 of /blocks/moodleblock.class.php: call to debugging()
line 584 of /blocks/moodleblock.class.php: call to block_base->has_add_block_capability()
line 236 of /lib/blocklib.php: call to block_base->user_can_addto()
line 1866 of /lib/blocklib.php: call to block_manager->get_addable_blocks()
line 1005 of /lib/blocklib.php: call to block_add_block_ui()
line 353 of /lib/blocklib.php: call to block_manager->ensure_content_created()
line 7 of /theme/rhubarb/layout/general.php: call to block_manager->region_has_content()
line 847 of /lib/outputrenderers.php: call to include()
line 777 of /lib/outputrenderers.php: call to core_renderer->render_page_layout()
line 243 of /course/view.php: call to core_renderer->header()
The exact same errors appear even when I switch themes even though the tinymce editor works in the other themes (such as Afterburner). So I'm not sure whether it's the block or the theme. I guess I'll try removing the block and see what happens.
If anyone has any other ideas I'd really appreciate it.