Preconditions:

• Defined field group in ibexa.yaml configuration file

 repositories
      default:
          field_groups:
              list: [content, admin only]
              default: content      

Steps to reproduce

1. Log in to back office.
2. Go to Admin / Content Types / Content.
3. Edit CT folder.

Actual result:

Defining field group as e.g. 'admin only' causes an error 500 while CT is edited.

The name "admin only" contains illegal characters. Names should start with a letter, digit or underscore and only contain letters, numbers, underscores ("_"), hyphens ("-") and colons (":"). 

Expected result:

User can edit CTs.

NOTE:
On v3.3 "admin only" works properly (see attached screenshot ezplatform.yaml file).
Currently for v4.4 and v4.5 we have to use field group name e.g "admin_only".