Purge caches again ... not just theme but site.
Two ways ...
1. edit config.php file and add a line to force the use of Clean:
$CFG->theme='clean';
OR
2. edit the mdl_config table for same variable.
Before doing the above, think I'd turn on debugging and see if there is anything reported.
Also check apache error logs for clues.
Since you upgraded, depending how you upgraded, something could be missing ... like 'bootstrapbase' ... OR
since you upgraded and depending upon how you upgraded, could be a owership/permissions issue on the new files.
So ... check apache error logs ... no clue ... turn on debugging .... nothing to report ... config.php file ... then get nasty and edit mdl_config table.
'spirit of sharing', Ken