Merge through Merge Request fails after upgrade

We upgraded gitlab today to 14.8.2 (from and older 13 version) and all went well except clicking Merge in any merge requests (whether created before or after the upgrade) is failing. All we get is this message on screen:
An error occurred while merging. Try again.

But there is nothing in the gitlab logs to indicate why it is failing. We’ve tried restarting the server and doing a gitlab-ctl reconfigure but nothing changes this behaviour. We’ve tried rolling back to older versions but we can’t get it to work any more. The rest of gitlab seems fine.

Any ideas how we can find out why it is failing?

This turned out to be a problem with the ‘hooks’ folder (under /var/opt/gitlab/git-data/repositories) for some of our repos. I rebuilt these based on a more recently created repo and the problem has gone away!