Gitlab-Ci fails to submit builds to coordinator

After the 8.0 upgrade, the CI fails build:

WARN[0347] 33ba87b6 13 Submitting build to coordinator... failed

GitLab shows the runners as alive, even gitlab-ci-multi-runner verify shows them.

I have no clue, what i've done wrong, what can be wrong or how to fix the problem.

Here is the output of gitlab-ctl show-config: config.conf