I have the problem described in the title (exactly as in this question).
While having the pdf viewer embedded there is no problem. When unchecking the embedded option, I cannot open the separate pdf viewer window, neither from the taskbar, nor with ALT+TAB. Both, while hovering the cursor over the taskbar symbol and with ALT+TAB the pdf viewer window appears empty but still cannot be accessed.
In addition to the other question on this problem, I have noticed: The problem occurs dependent on the connection to certain extension monitors (connected to my laptop to extend the screen). I am working with 3 different extension monitors (dependent on where I work for the day) and always the same laptop. Let's call the monitors: A (a flat screen TV), B1, and B2 (regular pc monitors). The monitors have different dimensions and resolutions. If the problem occurs on A, the B monitors don't have the problem and vice versa. I have experienced the problem on monitor A a couple of months ago and now on the B monitors (and now also while working on the laptop without screen extension, but not while extending with A). I have not changed any settings and am puzzled why this problem occurred in the first place and how it changed from the A monitor to the B monitors.
The problem persists independent of the content of the tex-file (so no minimum working code or try the code in the linked question). Compilation exited without errors and a pdf file to view exists. The problem occurred in Texmaker 5.0.2 and also after updating to 5.0.3. running on Windows 10 pro.
How can I get rid of the problem? How does screen extension or the characteristics of the extension monitor matter for Texmaker?