Genau, zuvor hatten die "Problemuser" die default-policy.
Hier die Werte der (bei uns) funktionierenden Policy, welche nun problemlos klappt:
AnonymousMaxConcurrency : 1
BookingSelfServiceMaxConcurrency : 10
ConsensusMaxConcurrency : 10
SchedulesMaxConcurrency : 5
EasMaxConcurrency : 10
EwsMaxConcurrency : Unlimited
ImapMaxConcurrency : Unlimited
OutlookServiceMaxConcurrency : 27
OwaMaxConcurrency : 20
OwaVoiceMaxConcurrency : 3
PopMaxConcurrency : 20
PowerShellMaxConcurrency : 18
PowerShellMaxTenantConcurrency :
PswsMaxConcurrency : 18
RcaMaxConcurrency : Unlimited
CpaMaxConcurrency : Unlimited
RcaSharedMaxConcurrency : Unlimited
DiscoveryMaxConcurrency : 2
PushNotificationMaxConcurrency : 20
EncryptionSenderMaxConcurrency : 200
EncryptionRecipientMaxConcurrency : 20
SuiteMaxConcurrency : 20