Errr. Firstly, which adapter are you using, and how is it wired? I think you’re trying to go from the PC via the VGA port > VGA-to-5-BNC cable > 5-BNC-to-SCART adapter > Lotharek Hydra – is that right? (if the adapter is a 4-BNC-to-SCART one, you’d just use your T-piece joiner again). The SCART cable should be setup for input, because it’s inputting the signal to the Hydra. You might be able to open it up and rewire it, or perhaps slide pins 19 and 20 out and swap them, if that sounds right. That said, if the Lotharek isn’t just passing the sync right through, you might have a problem. And VGA outputs TTL level sync (+5VDC) so while a PVM is happy enough to take 5Vp-p sync assuming nothing ever goes wrong, a SCART TV might not work regardless, IDK.
Regarding buying a new cable - most (VGA)HD15-to-SCART cables are only wired with H-sync, not V-sync. This is because composite sync would be carried over the H-sync pin, pin 13, and the cables were normally used to connect projectors to other gear, not for PC use. If do not think the ArcadeVGA can be made to output composite sync, you’d have to look into that. There are some VGA-to-SCART cables out there that will combine sync, or there used to be, like this one:
https://www.ebay.com/i/141663388712?chn=ps&dispItem=1Building a VGA-to-SCART cable isn’t impossible, if you know how to solder. Or, try contacting Andy from Ultimarc and asking for suggestions connecting an ArcadeVGA to a SCART TV. Or google it, and see what you think of the results