OBS has no idea what console is plugged into your capture card.
The Ripsaw (a re-cased and firmware flashed AverMedia LGX by the way, as the Ripsaw HD is an LGX2) is known to have a TON of weird issues and problems, and can be very finicky about the input it receives.
I'm not sure what resolution that Pound cable sends at, but if it's sending a resolution that the Ripsaw doesn't support, that could definitely cause the issue. Some hdmi-retro cables will send a fixed 720 or 1080 signal, others will try to pass 240i/p which is doubtful that the Ripsaw will be able to read. The Dreamcast is especially bad at this because even on VGA-Box-compatible games, the menus may be sent at a lower resolution, and some will 'force' non-VGA mode.
If it was me, I'd put an HDMI splitter between the cable and capture box, and send the other output to a TV to confirm signal and get past the startup and into game in actual-VGA mode. As an added bonus, no dealing with capture latency just playing on the TV.
tl;dr: the Ripsaw isn't a great capture box, and unsupported resolution shenanigans are likely due to Dreamcast being full of weird.