Bummer. I had that happen with the 600/11. I bought that lens in December 2020 and the R6 started to lock up to the point that I couldn’t shoot with it anymore. I returned the lens. Funny thing is that when I tried the EF 300/4L with the EF 1.4xTC, the camera locked up too. So I thought that apparently my R6 didn’t like that focal length. I never shot with that combination again and never had any lock ups since. Not sure what was going on. What I do know is that when I now try the 300/4L with the 1.4xTC, the camera doesn’t lock up so I gather that was a freak coincidence. But no idea why it started happening with the 600/11.
All I can do is “hope” it was a freak occurrence although my life experience is that if something happens once it’s likely to happen again. I prefer it never happens again.