Sounds weird but turns out to be quite simple ...
If you rename a Resource, there is an agent called "RenameReservations and SendNotice | Rename", processing all reservations affected by the resource name change within Resource Reservation database. Additionally, this agent sends a notice to every chairperson of a changed reservation, containg a button, which will change all affected Calendar entries in the user's mailbox.
So far, so good, but what ? You stringently use ECL ? Be prepared for some ECL alerts on your clients ! Why ?
The button, which is sent to the user is embedded in a document within the Resource Reservation database. Unfortunately this document is not signed with "Lotus Notes Template Development/Lotus Notes" but with the last updater's name, which for the German template is Hong. Gotcha !
From resrc7.ntf English
From resrc7.ntf German
For some reason, my customer's users are neither allowed nor willing to trust Hong ... (workaround is quite simple, just save the document with a valid user in your system).
Bug or feature ? ... yawn ...