Meanwhile I found out more about the problem.
1. all if I connect everything proper, I only see the Video, no OSD at all
2. If I disconnect Video-In, I can see the OSD
3. If i reconnect Video-In, the OSD disappears again -> video only
If you google for this problem, you will find some threads about ntsc/pal issues.
My camera is pal, so I switched in the config tool to PAL -> same behavior
So maybe my camera is labeled wrong and NTSC instead of PAL, so I changed the settings to NTSC -> same behavior
So I googled a bit more and found this thread
https://github.com/ShikOfTheRa/scarab-osd/issues/202 I'm using exactly the same camera "Eachine 800TVL CCD". Seems like cam's using "Nextchip 2090 DSP" are not compatible with minimOSD just older cameras with "Nextchip 2040" sadly you can not find ob banggood,ali, amazon and co which dsp the camera you are going to buy uses. Maybe we should introduce a compatibility list in the wiki and link it to the osd howto page. I just ordered three different cameras to try out which one might work.
also another sidenote: when I was able to see the OSD, the osd complained about not recieving any telemetrie data. I'm using OPLink with my revo board, I had to start the groundstation to recieve the telemetrie data, only when oplink stuff was connected, i was able to get the data in the osd as well.