Possible theme['path'] issue when updating to Drupal 11.4

Problem/Motivation

After updating a website to Drupal 11.4 and PHP8.4, I've been getting open_basedir errors and (parts of the) the generated subtheme won't load.

After some digging it seems that $theme['path'] is not being set correctly. Because of that some checks for the theme version fail because those are based on the presence and location of specific folders.

I'm running version 7.1.1 of AdaptiveTheme. That version was also installed since March (and working fine), so that hasn't been updated. The subtheme was also created in March.