Go to the Grader report and click on aggregates only.
The following error message appears:
Coding error detected, it must be fixed by a programmer: Invalid value in set_user_preference() call, value is too long for the value column

The log file gives me the following information:
Default exception handler: Coding error detected, it must be fixed by a programmer: Invalid value in set_user_preference() call, value is is too long for the value column Debug: \nError code: codingerror\n* line 1789 of /lib/moodlelib.php: coding_exception thrown\n* line 1636 of /grade/report/grader/lib.php: call to set_user_preference()\n* line 110 of /grade/report/grader/index.php: call to grade_report_grader::do_process_action()\n, referer: http://moodle.htl-kapfenberg.ac.at/moodle/grade/report/grader/index.php?id=469

Thx