when starting a simulation from a subst location on windows, runner.py line 154 calls relpath which
in turn calls ntpath.py leading to the following:

Traceback (most recent call last):
File "C:\Users\mss7pk\AppData\Local\Programs\Python\Python38-32\lib\site-packages\vunit_hdl-4.4.0-py3.8.egg\vunit\ui_init_.py", line 726, in main
all_ok = self.main(post_run)
File "C:\Users\mss7pk\AppData\Local\Programs\Python\Python38-32\lib\site-packages\vunit_hdl-4.4.0-py3.8.egg\vunit\ui_init.py", line 772, in _main
all_ok = self.main_run(post_run)
File "C:\Users\mss7pk\AppData\Local\Programs\Python\Python38-32\lib\site-packages\vunit_hdl-4.4.0-py3.8.egg\vunit\ui_init.py", line 808, in _main_run
self.run_test(test_list, report)
File "C:\Users\mss7pk\AppData\Local\Programs\Python\Python38-32\lib\site-packages\vunit_hdl-4.4.0-py3.8.egg\vunit\ui_init.py", line 988, in _run_test
runner.run(test_cases)
File "C:\Users\mss7pk\AppData\Local\Programs\Python\Python38-32\lib\site-packages\vunit_hdl-4.4.0-py3.8.egg\vunit\test\runner.py", line 120, in run
self._run_thread(write_stdout, scheduler, num_tests, True)
File "C:\Users\mss7pk\AppData\Local\Programs\Python\Python38-32\lib\site-packages\vunit_hdl-4.4.0-py3.8.egg\vunit\test\runner.py", line 154, in _run_thread
print("Output file: %s" % relpath(output_file_name))
File "C:\Users\mss7pk\AppData\Local\Programs\Python\Python38-32\lib\ntpath.py", line 703, in relpath
raise ValueError("path is on mount %r, start on mount %r" % (
ValueError: path is on mount 'D:', start on mount 'W:'

A quickfix is to remove line 154 completely but probably there is a more elegant solution to this behavior..