I have fixed the issue. From the logging above you can see that it errors after requesting the sys-nrf-role objects. These objects are in the DAL.
cn=sys-nrf-role,cn=EntityDefs,cn=DirectoryModel,cn=AppConfig, cn=UserApplication,cn=IDVAULT,ou=Identity Management,o=System
Given the fact that it tries to resolve String, Locale i suspected that one of the locales would not be present or something. I therefore made a backup of the XmlData attribute of the object. Copied the descriptions from a working environment... and the role display is working again.
So, for some reasons this attribute was not updated with (new?) languages or somehow got corrupted. It's working now !