DAVx5 Nextcloud Calendar invitations

Dear all,
I am using NC since some 7 years now. It grows and improves. Since a while i use NC even for contacts and calendar.
Now i installed a new user for a friend on my server and connected this user in the same way as my own.

if that user creates a calendar invitation on his phone, there are no messages sent to invitees.
if my user creates a calendar invitation from my phone, messages are sent correctly.

both mobiles latest LineageOS, Nextcloud app, stock calendard app
both users with email adresses configured in NC account

inviting via web-gui from NC calendar sends invitations for both user accounts.

I downloaded DAVx5 Debug log and reviewed them. In the failing client there ist a hint for the to-be-sent invitation, but no errors.
β€œical4android: PARAMETER #0 = attendeeEmail=[xxx]@gmail.com attendeeName=[…] attendeeType=1 attendeeIdentity=null attendeeRelationship=1 attendeeIdNamespace=null attendeeStatus=0”

in nextcloud.log there is no information at all about that sync (so no error).

anybody an idea where to look next ?