I find out why I got the mapi error during login in the webapp. For all of my created users no store was automatically created, too.
root@kopano:~# kopano-cli --list-users
User list for Default (3):
User Full Name Homeserver Store
Brigitte Brigitte kopano
SYSTEM SYSTEM kopano 8962FFEFFB7B4D639BC5967C4BB58234
Uwe Uwe kopano
After adding the store manually the logon is working successfully.
root@kopano:~# kopano-cli -u uwe --create-store
root@kopano:~# kopano-cli -u brigitte --create-store
root@kopano:~# kopano-cli --list-users
User list for Default (3):
User Full Name Homeserver Store
Brigitte Brigitte kopano 227160E136164EC7835FEAAA20E622CC
SYSTEM SYSTEM kopano 8962FFEFFB7B4D639BC5967C4BB58234
Uwe Uwe kopano 4183C8086C584490A54C2E816AA5E840