Your PHP installation needs the 'DOM' library added. It's unrelated to PHP version.
Vanilla silently added a few library dependencies like that over the last couple years without precautions for existing open source users. I do not know if they have a plan to remedy it.
This is the current list of required extensions (lifted from composer.json):
Nikita4ka
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See reply to you other post.
I see you changed from 3.3 to 2021.012
Use this instead
https://open.vanillaforums.com/addon/vanilla-core
.012 is not considered stable.
Also you php changed from 7.3 to 7.2
Might need that back at 7.3
Your PHP installation needs the 'DOM' library added. It's unrelated to PHP version.
Vanilla silently added a few library dependencies like that over the last couple years without precautions for existing open source users. I do not know if they have a plan to remedy it.
This is the current list of required extensions (lifted from composer.json):