I had the same on Windows. My solution was to get which port the debug wants to connect to. (In IntelliJ a red rectangle already giving the info: "Error running Tomcat: Unable to open debugger port (127.0.0.1:XXXXX): ... Already in use...") Let's say XXXXX is the port number. Then i searched for the problem and the PID in a cmd window:
netstat -ano | find "CLOSE_WAIT" | find ":XXXXX"
I got the PID number as the last number in the result line. (Let's say YYYY) Finally:
TASKKILL /PID YYYY
An extra info: Winscp logged out meanwhile, probably it was causing my problem. :)