Hi Dimitri
Thanks for your submission. This is not a defect of the LN-Teammail Template. This happens all the time, if you are updating a MailFile with a new template - even with official templates and happens with existing Folders. You have to manually update the folder design. There is an agent for you under "Aktionen - Administration - Ordnergestaltung aktualisieren" to do that. Alternatively, you may delete and rebuild the folders.
The reason for this "error" is: custom folders will not be updated by a design change, hence, they have still the old code and are trying to run routines from the old scriptlibraries, which obviously are no longer available after the update.