I am happy to report that this dual screen bug has been finally solved 
To upgrade, please download the new XModule installer V202009b for Windows. It upgrades the Real User simulation XModule to V1.0.21. There is no update for Mac and Linux, because this bug was always a Windows only problem. Note that the RPA browser extension itself does not need to be upgraded.
What does this fix do?
This upgrade solves the problem that XClick mouse clicks hit the wrong area when a second monitor is connected. It also allows you to send mouse clicks to the “not-main” monitors if you know the x/y coordinates (e. g. XClick | -300, 20)
What does this fix not do?
Computer vision still searches “only” the main display. So it can not find apps or buttons on other displays. A further upgrade that adds multi-screen image search is planned for 2021 if there is demand for this feature (=> if you need it, please open a forum post for it).
Today’s upgrade should solve all of the problems reported in this thread. But of course, if you continue to have problems with multi-screen configurations, please let us know.