So... I said it was solved. Not really.
Over the past month, I was playing mainly with my RST-135 mount (which I need to setup/align every night) and 1.6.0.
There I had this error maybe a couple of times but then it just went away - everything was running smoothly for about 10 nights in a row.
Everything was running on RPI.

Tonight I switched to my permanent installation (HEQ5).
There I don't normally do Polar Alignment so I didn't notice before but when I tried I got this "FOV must be 10 arcmins" issue and I couldn't get rid of it!
I restarted/rebooted several times. Rearranged cables. Nothing helped.

I reverted back to 1.5.9 and there it worked.
On RPI.
When switched to Windows KStars client (latest stable 3.5.5) - I again got "FOV must be 10 arcmins" error.

Well. Hopefully it will get sorted out in the next release.
It's really behaving strangely.

Just reporting.
Thanks!

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