Black window on multi monitor setup

Hi all
I have seen an odd thing a few times.
When a few of our customers op our main desktop app and they have more than one monitor and the program starts on monitor 2 or 3 (not primary) it will have a black window. I can see the app name and the system buttons (close, maximize, minimize). But thats it.
When I minimize and maximize again the program pops up on monitor 1 again and is back to normal.
I use Clarion 11.1.13845, a bunch of Capesoft accessories and a few IceTips accessories.

Any idear?

Is this an MDI or non-MDI app?