Hey mnwr,
it seems that you have the same problem than me (here was my case: Virtualbox 7.0.8-2). I began to think that i was alone with the problem.
For me, the hint from @coolfreezer did the job. I had to do:
- sudo nano /etc/default/grub
- edit the line “GRUB_CMDLINE_LINUX_DEFAULT”
- and insert the entry “ibt=off”
After a reboot all my virtual machines worked, even with the extension pack installed. But be carefull! I am still searching what i turned off with this entry.