Finally, I uninstalled dotCover because the "trial license" was long expired and the "Please register" dialog box caused both hassle and problems. The Add-in did not load without license, so it couldn't be disabled from the GUI.
Now, Resharper wont run unit tests. Not from unit test explorer, not from hotkeys, not from test session, not from code sidebar. Nothing.
So, if you are in the same situation, do not uninstall dotCover, but disable it from the ini-file.