I'm having the following problem, I'm getting the above message and frequently it takes ReSharper over 10 seconds to unload after all my unit tests have already finished.
My reproduction scenario consists of running a single NUnit test under debug. Normally the test finishes (I see the UI stops spinning and the process closes). However, if I put a breakpoint in the code and then let it continue, the test finishes (UI stops spinning, test fixture one time tear-down is done), but the process takes over 10 seconds to stop and the above message pops up.
I paused the process during this hanging phase and below you can see the remaining threads.
ReSharper 2018.1 build 2018-04-14