Sometimes the eM Client window won’t minimise. At other times when I have minimised it, it spontaneously re-opens and then won’t minimise.
I’ve discovered if I go into the Window menu tab and click on Main Window it will replace the un-minimisable window with a normal one that will minimise.
On my iMacPro (M1 processor) this happens occasionally, on my McBook Pro (Intel processor) is happens consistently.