Another instance of Audacity running error

I’ve been running audacity for a long time on a Macbook pro using High Sierra without problems but today, after downloading some system upgrades, I can’t run Audacity. I get a window popping up which says there is another instance already running and that I should use it to open files. BUT there is no other version running.
Even after I Force Quit what shows in the system monitor, then restart Audacity, this message reappears, and again there is an icon showing it running in the system monitor. I force quit that, try to restart and the same thing occurs. How do I get this up and running again?

Have you tried restarting the Mac?
If that doesn’t work, go to ~/Library/Application Support/audacity and trash the contents of the “SessionData” folder.
– Bill

Thanks Bill. Restarting didn/t work and I even reinstalled Audacity without any improvement, but erasing the contents of the session contents folder did the trick.