Problem: the IntelliJ Debugger doesn't stop at any breakpoints in Java code.
Details: using version 2020.3. It was fine last week but it could be there was an update over the weekend that caused this problem.
What didn't work: Unmuting breakpoints, disabling then enabling breakpoints, restarting IntelliJ after Invalidating cache, deleting .idea folder.
Eventually I solved it. See solution below.