TYPO3 Core

This looks a bit fishy at first glance, afaik this install tool part should not load "TYPO3\CMS\Core\ExpressionLanguage\Resolver" at all.
Could you provide the full backtrace, please?

Did you by chance already try to flush caches? In that state, it's very likely you have to flush caches manually on the filesystem. Depending on the installation mode, the cache directory is either public/typo3temp/var/cache or var/cache.

I have flushed caches several times.

@Gratian Alinei

Looking through the attached "backtrace.pdf" it looks like the l10nmgr
is interfereing here in a event and try to loading a site for some
language constraints. And if the "yaml" file contains conditions (e.g
baseVariants) it loads the expression stuff and so on.

However, you are reporting this for TYPO3 v12 . looking into the (public)
repository and the TER there is no TYPO3 v12 compat version as far as I
can find.

Please try to remove l10nmanager and check if the error still occurs. If
yes it may be unrelated, if not - report it to the l10nmgr maintainer
referencing to this issue (and dropping here a reference to the issue in the
repository then).

Maybe, this pull-request in l10nmgr may help: https://github.com/CodersCare/l10nmgr/pull/31
Not tested thou.

If it works without l10nmgr report the issue here: https://github.com/CodersCare/l10nmgr/issues

I had the same problem in TYPO3 v12 and can confirm that it works without EXT:l10nmgr installed.