Hello,
We've also recently upgraded to Moodle 2.6.x using git from MOODLE_24_STABLE -> MOODLE_25_STABLE -> MOODLE_26_STABLE and encountered this problem.
The SQL script supplied by Johan appears to have corrected the issue for us however.
Thanks,
Dan Jackson (Lead ITServices Technician)
Long Road Sixth Form College
Cambridge, UK