Thank you, Nebulon, for your reply.
The runners are on a different machine.
I don't think that the machine with the runners is running out of space because it has a very large amount available on all partitions (the root has 300GB available). I think the error is originating from the cloudron machine (although I may be wrong).
In order to try to confirm the location of the error, I will setup another gitlab runner client on a different machine, and see if that has the same problem. Hopefully that will isolate the error either to the Cloudron server, or else the gitlab runner machines.