RTG behaviour with I/O-Board and VGA

[DoubleA]

Trying to configure RTG, but so far without success. Using a BENQ TFT with native Full-HD resolution and VGA.

My (relevant) .ini settings are:
vga_scaler = 1
forced_scandoubler = 1
vscale_mode = 2 ; which is essentially 1, because of the doubled resolution from forced_scandoubler
video_mode = 9 ; Full-HD PAL (there is no difference to mode 8 Full-HD NTSC in the end)

Native Amiga screenmodes work fine and are crystal clear. But Mister RTG modes scale very strange (with 640x480 as an positive exception, that just scales to 1280x960 centered on screen integer x2).

1920x1080 Mister mode e.g. opens a strange 1580x480i screen which is interpolated afterwards to Full-HD (and therefore is completely unreadable).
And all other RTG modes than 640x480 produce a guru meditation sooner or later.

Any idea what i'm doing wrong?

Thanks in advance for your help.

[attle]

I have exactly the same issue. But to clarify, I don't think it's strictly related to IO board and VGA. I use HDMI output on my 1080p Dell screen.