by Pavel Krejci.
This is most likely caused by a custom theme which is not fully compatible with Moodle 2.6.
Easiest fix is to temporarily add:
$CFG->theme = 'standard';
to the bottom of your config.php file.
See MDL-43214 for more info. Developers decided to blame it on the theme and not to fix this situation. Even though with just a little bit out-of-the-box thinking this could be easily resolved.