Thanks to your thoughts, I realized that also a few completely different processes took much longer than expected, like deleting a course or a user. Subsequently, server error message 500 brought me to the error logs, where I identified the line:
[error] PHP Fatal error: Cannot redeclare questionnaire_upgrade() (previously declared in /..../htdocs/moodle/mod/questionnaire/db/mysql.php:3) in /..../htdocs/moodle/mod/questionnaire-muell/db/mysql.php on line 76
Explanation: before changing something in questionnaire module, I had copied the /questionnaire directory to /questionaire-muell. The pure existence of this directory triggered a pending and faulty module installation process.
I suggest you set up a forum for the subject "Mistakes and silly misconfigurations never thought of "