I got the same error. I'm updating a bunch of sites for different clients (all with the same hosting provider). So far I discovered the following:
The error only comes up when the Helix Ultimate 2.0.2 update was performed on Joomla 3.10.1, so before the upgrading to Joomla 4. When I then switch back from php 8.0 to php 7.4, the template backend is working fine.
When I first upgrade to Joomla 4 and then update the Helix Ultimate template, I don't get an error.
Don't know what's causing it. A fix so I can enable php 8.0 again would be very welcome. ;-)