Could it be that your Fiji crashed recently? If so, you can likely find a file called
xxxx is your user name) in your temp directory (usually
C:\Users\<your-user-name>\AppData\Local\Temp on Windows; just enter
%TEMP% in the address bar of Windows Explorer).
After making sure no ImageJ or Fiji process is running, try deleting the file and restarting Fiji.
The issue is related to the single instance listener option, see this archived bug report and also some related discussion here: