7 – HTML tags after umlauts causing premature header sending – CK editor

I’m maintaining some old Drupal 7 site (not created by me) and since couple of months ago it started acting strange (I didn’t make any change on site):

Inside some content type there is a formatted text field that is edited with CKEditor.

If field contains just a plain text and I try to save it (when editing the node) it works well.

If I make some text bold at beginning of the whole text and try to save it works well also.

But if I try to make some text bold after first umlaut character (site contains German text) and try to save I get 403 error – forbidden?!?

In drupal’s error log I see the error:

Warning: Cannot modify header information - headers already sent by (output started at /home/www/project_name/includes/common.inc:2776) in drupal_goto() (line 705 of /home/www/project_name/includes/common.inc).

I even suspected that site has been hacked but I don’t see any suspicious new files there.

Any idea?