Hello Hartmut,
I actually got this response to this problem and I tried it and it worked for me. The "content" option is now back in my drop-down menu for page region!....so I figured I'd pass it on to you:
The 'content' block is a fake block added by code and often does not show up, so at a guess what I think you need to do given that you have added it back and stipulated a position for it to be is in the database:
- Edit the 'block_instances' table and find the row for the 'blockname' of 'course_overview'. Note down the 'id' and change the 'defaultregion' to 'content'.
- Edit the 'block_positions' table and remove any rows where the 'blockinstanceid' is the same as the 'id' above.
Then 'Purge all caches'.
This is only a hunch, so if the DB contents are important, make a backup.
Original source: https://moodle.org/mod/forum/discuss.php?d=258075#p1119019
(Edited by Mary Evans - original submission Thursday, 10 April 2014, 7:00 PM)