Failed to set the videomode

I'm grading a short piece shot in 3840x2160 at 50 fps, Apple ProRes 422HQ.
My system is a 27" iMac i7 with 40 Gigs of Ram, Resolve 18.
How do I set up project settings and the BM Desktop Video Setup to display video on my external monitor (Panasonic BTLH 2550)? I keep getting the dreaded "Failed to set the videomode on the video I/O device." dialog box in Resolve...
The reference video, which is also 50fps, but 2560x1440, H264 displays fine on the external monitor. What am I not getting?

What I/O device are you using? Failed to set videomode seems to suggest that your I/O doesn't support the monitoring set up.

What I/O device are you using? Failed to set videomode seems to suggest that your I/O doesn't support the monitoring set up.

Oh man, I should have mentioned that! I’m using the BM Ultrastudio Mini Monitor.

Yep, that doesn't support handling a 4k50 signal (nor does your monitor)

As that's a 2K/HD device you can set your timeline resolution to HD.

If you need to render to UHD, change it back before rendering.

Thx, that did the trick!