OK, UserForm is opening on the monitor where VBA editor was last time used(closed). Usually when we do some project we place excel on one monitor and editor on the other for more visual space. After work you save and close editor. But VBA remembers that position, but not the position of editor exactly, more like position regarding to Excel. So, if you open excel on either of the monitors, userform or editor opens on other monitor! That's how this issue begins to happen!
So easiest solution without any code, is to open editor(Alt+F11) and drag it to the same side as excel is, and close it. Than open userform.