Recent Builds fail on self-hosted

For a few weeks now, all builds on self-hosted fail. This concerns all jobs of gitlab-opentofu-image:build no matter the version, for both alpine and debina

All of the jobs fail with the same message when configuring buildah it seems:

$ if [ -n "$CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX" ]; then # collapsed multi-line command
Detected GitLab Dependency Proxy at 'git.corp.net:443/devops/ci-cd/dependency_proxy/containers', configuring it for buildah ...
Error during unshare(CLONE_NEWUSER): Operation not permitted
time="2025-07-07T08:48:06Z" level=error msg="parsing PID \"\": strconv.Atoi: parsing \"\": invalid syntax"
time="2025-07-07T08:48:06Z" level=error msg="(Unable to determine exit status)"

Operation not permitted sounds like an issue with unprivileged runners, but that's not the case here. I'd appreciate some advice cause I cannot really find the root cause here.