Clone/Pull RPC failed

we are running GitLab CE on Version 11.9.1 in a Kubernetes Cluster and since the update to the latest version we are facing the following problem:

On pulls, fetches and clones, locally and in runners often we get the error:

error: RPC failed; curl 52 Empty reply from server
fatal: the remote end hung up unexpectedly

This doesn’t always happen but most of the time.
What we do: You just run git clone, wait for 20-30 seconds and then get the error.

We have checked the logs, firewalls and reverse proxies and didn’t see any errors there.

GitLab worked the whole last year without any problems until friday.

Do you have any clue what we can do?