Drupal\field_permissions_group\Plugin\FieldPermissionType\CustomGroupAccess::create(Object, Array, 'custom_group', Array, Object) (Line: 58)

Found another related issue with 📌 Add compatibility with Group v2 and v3 Fixed with a partial fix. Might be as easy as making the same change from plugin.manager.group_content_enabler to group_relation_type.manager https://git.drupalcode.org/project/entitygroupfield/-/merge_requests/4/d... and

use Drupal\group\Plugin\GroupContentEnablerManagerInterface;
use Drupal\group\Plugin\Group\Relation\GroupRelationTypeManagerInterface;

jayelless → made their first commit to this issue’s fork.

Merged to branch 2.0.x.

Thanks for your help everyone!