Ok so the problem is that coverage calls os.path.relpath(filename) with filename=D:\\a\\tmp_folder\\.coverage.fv-az639.3520.998466 while the current working directory (at least in the subprocess) is in the site-packages folder on the c:\\ drive:

cwd        = 'c:\\miniconda\\envs\\testvenv\\lib\\site-packages'

Hence the ValueError raised by relpath.

The fact that the cwd is in site-package is probably a side effect of pytest --pyargs, e.g. to have the conftests.py.

I will open a PR to make coverage robust to this case. But this is weird that it does not crash in general (that is for the coverage file that are generated from the main process).

Upstream fix seems to work as expected. Closing debug PR.