Tests job failing with error "child process failed, exited with 48"

The tests job fails sometimes with the error:

$ mongod --fork --syslog
about to fork child process, waiting until server is ready for connections.
forked process: 921495
ERROR: child process failed, exited with 48

Inspecting the node with the gitlab-runner one can see a leftover mongod process running:

 947525 ?        Sl     0:59 mongod --fork --syslog 

After killing the process and restarting the job (so that it is executed on the same runner) the job succeeds. But one gets the warning message:

WARNING: Cleanup script failed: unknown Custom executor executable exit code -1; executable execution terminated with: signal: terminated

and again a mongod processs is left over. Obviously the mongodb instance could not be stopped and it was also not killed by enroot.