Thanks for reply. Luckily I am now on clear waters.
Debugging showed that ...theme/fordson/classes/output/core/course_renderer.php -file was somehow invalid. "Settings Tab-menu" was missing also on other pages, e.g. adding students.
My guess is that updating Fordson theme to 3.7 rel 1 somehow damaged the php-file.
The fix was that I uninstalled Fordson totally and installed it again. Now everything works OK again.