Well after writing my previous posts I've been diving, and had the surprise to see both strobes were failing (either not shooting, or shooting too weak), whereas I would have expected the strobe connected with the right FO to behave well!
Then later in the dive all went back to normal, meaning both strobes worked perfectly, be it in TTL or manual, even the left one (the one which failed to trigger strobe on land).
Very strange! But anyways I've ordered two new FOs (35 EUR each), will shoot with them and see if I have any issues.
I'll report my findings here after a few shooting dives.
Thanks again for all the advice!