video output on 8250

Did you try a different SCART cable already ?
I know that my 8255 needed a crossed cable for some reason, but later I modified it so I could use a normal cable.
Or check the soldering of the SCART connector in the MSX, maybe a loose contact... Can happen after several years.

I tried 5 random different SCART cables on my nms8250/00 and got no problems here.

You can try another SCART cable or try another device on your current SCART cable (dvd player or something) to see if the cable works on another device.

If nothing works then i suggest what SkalTura is saying about checking the soldering.

The SCART always has "crossed cables", i.e. it has dedicated output and input pins for audio, composite/S-Video/RGB (EDIT: seems I misremembered, this is not true for RGB). There is only one kind of SCART cable (per signal type; not all might be fully connected, i.e. there might be an audio+CVBS only cable, for example), a "straight SCART cable" is just wrong. I've played a bit with the SCART/AV portion of my NMS8255 and other devices with RGB scart, and the pinout is exactly as per SCART spec (RGB + audio output only).

There are two peculiarities with the NMS8250; the composite video output only has clean composite sync (with no video signal). Then there's something with the blanking signal; I'm not sure I remember correctly from the top of my head, but IIRC the blanking signal is wired a bit weirdly on the NMS825X: It is connected straight to 5V, although it should be 1-3V as per spec. Most TVs are fine with 5V but some actually check if the voltage is above 3V and have problems with that! Someone who has their NMS825X fired up and ready might verify/measure the blanking pin(16) level.

The RCA outputs should output bog standard composite + mono audio, which should work on practically anything with composite input.

I'm not sure how one can get a B/W picture from the NMS8250. It is possible the NMS825X it is just broken.

I'd verify first the monitor/TV can accept PAL composite video. (note, the NMS825X has three RCA connectors: luminance, composite(VIDEO) and audio). After composite works, try to get RGB working. Check the SCART cable is indeed RGB, not all of them are.

https://pinoutguide.com/Audio-Video-Hardware/Scart_pinout.shtml
https://en.wikipedia.org/wiki/SCART

@Manuel: The video signals In/Out were swapped (pins 17 to 20), and the audio In/Out (pins 1,2,3 and 6) also.
No idea why this was done....

I have never had any issues with fully wired scart cables, but I only use RGB.