This is caused because two files in 7.x no longer exist in 8.x
Thus, a fresh install is OK, but if you upgrade over an existing 7.x instance, you will have a mix of some 7.x and 8x files.
This is normally not an issue, because the files are just ignored. But in this case, Tiki scans the whole directory...
In your error logs, you will get an error like this:
Fatal error: Call to undefined method TikiLib::list_slide_styles() in lib/prefs/slide.php on line 12
Fatal error: Call to a member function list_all_groups() on a non-object in lib/prefs/wikiapproval.php on line 28
Developers didn't notice because when they do "svn up", deleted files are removed. (http://dev.tiki.org/Get+code)
Solution is to delete the two following files:
To avoid others having issues, I have added some blank files that will be part of 8.3