Runner unable to access via gitlab-ci-token: "Empty reply from server"

Hi,

we have an OnPrem Gitlab and a Windows Runner to build our project. This has worked for the last year.
Since this morning, for some reason our pipelines fail when the runner tries to checkout the branch with
fatal: unable to access 'http://gitlab-ci-token:xxxxxxxxxxxxxxxxxxxx@gitlab.company-internal.de/OurGroup/subgroup/Our.Repo.git/': Empty reply from server

The log always goes

Fetching changes...
[...bunch of fetched changes...]
HEAD is now at [commithash][commit message] 
fatal: unable to access 'http://gitlab-ci-token:xxxxxxxxxxxxxxxxxxxx@gitlab.softfair-local.de/finanzlotse/fl3/Softfair.Finanzlotse3.git/': Empty reply from server
Checking out [hash] as [branch]...

And then errors depending on what the job actually tries to do. Currently one goes

ERROR: Downloading artifacts from coordinator... error couldn't execute GET against http://gitlab.ourcompany-internal.de/api/v4/jobs/311133/artifacts: Get http://gitlab.ourcompany-internal.de/api/v4/jobs/311133/artifacts: EOF  id=311133 token=r9He_F5q

a few times until it fails, another (our build job) goes

fatal: reference is not a tree: ecd08ae98716edbe7d6d344ba711b7eb15b9e147
ERROR: Job failed: exit status 128

We can still clone the repo locally. Just the runner fails. We can also build the javascript part in our linux docker runner. Just the windows runner seems to have problems.

Hope you can help us, I’d like to go to christmas vacation without the company gitlab being broken.

Greetings,
Marlon Regenhardt

Update: After rebooting the server the gitlab runner runs on, it works once again.