Same here with the freezes/lock-ups - even after the firmware update. This was with 400III+2x, with the 400III also having had its firmware updated to the latest. Screen and EVF go black for what feels like 10-15 seconds, and then it seems to resolve itself. That makes it no less annoying when it happens though - in my case, right in the middle of a BIF sequence when the birds where at the optimum position.
Canon - fix this ASAP - this is not acceptable on a 4K premium body. I have to say though, that once or twice (over a 6 month ownership period), I've had the 1DX3 also lock up, requiring me to remove the battery and reinsert it. But with the R5 this seems to happen several times a day, not a few times a year.