custom_hook return code no longer recognized

Since today the GitLab update hook does suddenly no longer works - pushing a new branch succeeded even if the hookd was a simple script like

#!/bin/sh
#
exit 1

What helped was to delete the affected branch at the command line at the GitLab server. Deleting that branch at the WEB GUI won't work. House Keeping did not show an effect. And it did not work to delete the branch at the client side and to do a forced push.

We do run here version 10.6.4.