Comment 8 for bug 1881375

If such were possible, that'd be helpful. No file referenced. Error is:

ERROR: Failed to start viewer: [Errno 48] Address already in use

Traceback (most recent call last):
  File "site-packages/calibre/gui2/viewer/main.py", line 202, in main
  File "site-packages/calibre/gui2/viewer/main.py", line 144, in ensure_single_instance
  File "site-packages/calibre/gui2/viewer/main.py", line 118, in create_listener
  File "lib/python2.7/multiprocessing/connection.py", line 132, in __init__
  File "lib/python2.7/multiprocessing/connection.py", line 256, in __init__
  File "lib/python2.7/socket.py", line 228, in meth
error: [Errno 48] Address already in use

I found a /Library/Caches/calibre-singleinstance-501-calibre_viewer.lock though that doesn't help.