I'm testing this issue since December and now I have two boxes to freely test, I can be sure (after a lot of tries, either with a box already "equipped" or with another when I started from scratch) it happens:
1) Only if you have VPS installed.
2) It now happens only on Ultimo4K and not also in Solo4K after the last VPS update.
3) Oscam has nothing to do with it: I tested it even for more than 24 hours, during which the box was on Rai1HD.
But the problem occurs ONLY on those RaiHD channel which have gone FTA since 14th December 2021, probably temporarily: Rai1HD, Rai3 HD, RaiSport+HD and others.
So, it's a paradox: Rai has no VPS system but VPS causes problems only on those channels.
In all other regularly encrypted Tivusat channels, I've never had any freezes.
When I found out this fact, I went back and forward many times to have confirms, and it is so.
I'm now testing a different VPS binary a friend sent me but, by this binary it is not able to record in "pure" VPS mode for those channels, like the Swiss ones, where it has always worked.
I'm now trying to be sure if, at least, this different binary is able to work at least in safe mode.
But, after hundreds of tiring tests, I'm now sure it's the VPS plugin the responsible.
As a final word, in case you wonder, the freezes happen even if there is no timer set on those RaiHD channels mentioned above.
Of course, there are other timers set for other channels, maybe, but the freezes occur even when they are not running.