Moodle in English

When trying to create a new wiki, I get the following error:
===
error/moodle/invalidcoursemodule
The ID for the course module is invalid. Please contact the site admin.

Debug info: SELECT id,course FROM {course_modules} WHERE id IS NULL
[array ()]
Error code: invalidcoursemodule

Stack trace:
    line 1451 of /lib/dml/moodle_database.php: dml_missing_record_exception thrown
    line 1427 of /lib/dml/moodle_database.php: call to moodle_database->get_record_select()
    line 7116 of /lib/accesslib.php: call to moodle_database->get_record()
    line 1052 of /mod/wiki/locallib.php: call to context_module::instance()
    line 278 of /mod/wiki/pagelib.php: call to wiki_delete_locks()
    line 130 of /mod/wiki/pagelib.php: call to page_wiki->process_session_url()
    line 885 of /mod/wiki/pagelib.php: call to page_wiki->print_header()
    line 118 of /mod/wiki/create.php: call to page_wiki_create->print_header()

Output buffer: <b>Notice</b>:  Trying to get property of non-object in <b>/var/www/html/moodle/mod/wiki/locallib.php</b> on line <b>1051</b><br /><br /><b>Notice</b>:  Trying to get property of non-object in <b>/var/www/html/moodle/mod/wiki/locallib.php</b> on line <b>1052</b>
===
Moodle 2.7.19 (Build: 20170313)
LAMP, PHP Version 5.6.30-0+deb8u1, MySQL 5.5.54

It had been working the last time tested, a couple of weeks ago. In between there was only the security update on the 19th.

This is reproducible. No new wiki is possible. Has this been reported?

Edit: Panik! Even existing wikis can not be opened in the whole site. Always the same error!
:-(

Edit 2: Another suspicion is that it started after deleting a wiki which had sub-pages. A known problem?