RetroScaler GBS-C and a GX4000

[VanZan]

I have the AliExpress/RetroScaler GBS-C (the blue one). Works fine with a PS2. The GX4000 is totally unstable with it syncing every few seconds. Just cannot get it going reliably. 

Is it because of the SCART cable? The PS2 cable is a Luma one and the GX4000 is a FLAM electronique one. According to ChatGPT the FLAM cable won't have CSYNC which could be the problem. 

My GX4000 has the RF switch and RF jack at the back which probably means it's not the French one.

Any help most appreciated!