Problem solved: as it often happens with IT and software, the most illogical action was the decisive one
It had nothing to do with loss of signal because:
1) I noticed that the same channel was perfect under BH and it was perfect in the same transponder for the same provider but for other channels.
2) Because it always crashed EXACTLY after two seconds following the zap, and before the crash I could watch the channel (if a signal loss occurs, it's not always at the same moment but it occurs randomly).
As the problem concerned only two channels, I tried to zap to them not within the bouquet I had put them in but from the whole list of channels and... surprise! No problem from there.
So I just deleted these 2 ones from the main bouquet I use and added them again and now it's OK.
This means that some infos about these two channels might have been got somewhat corrupted only in the bouquet.
P.S. The two channels were Tivusat ones, regularly decrypted by my card+cam Tivusat but the issue, in this occasion, had nothing to do with that.