[1.26.x] all gitlab-runner jobs failing due to filesystem permission errors

Since gitlab-runner-operator updated itself to 1.26, all our Gitlab CI jobs fail with filesystem permission errors like:

/scripts-43-19601/step_script: eval: line 174: can't create /kaniko/.docker/config.json: Permission denied

We deleted our whole gitlab-runner namespace, reinstalled the operator into it (which wasn't possible in 1.25), tried 1.26.0, 1.26.1 and 1.26.2 but no version works.

This also happens for service containers.

Please advise how to debug and fix this, thanks!