cmillerLCEMaybe try and perform a Set-App operation on the Webex Add-In. We do also have that Add-In and I was able to produce a Powershell error (not the same you got), though the user was still able to manage Add-Ins.
[PS] C:\Windows\system32>get-app -OrganizationApp -Identity 7a91e319-a65d-4ceb-909b-12203561dbf5 | fl DisplayName,ProviderName,UserList
DisplayName : Cisco Webex-Funktion zum Ansetzen
ProviderName : Cisco Webex Inc
UserList : {domain.com/Users/testuser}
[PS] C:\Windows\system32>Get-App -Mailbox testuser
WARNUNG: Unerwarteter Fehler. Ein Watson-Abbild wird generiert: Deserialization of type System.MarshalByRefObject blocked due to InDeny at location ClientExtensionCollectionFormatter..
Eine Ausnahme vom Typ "Microsoft.Exchange.Diagnostics.BlockedDeserializeTypeException" wurde ausgelöst.
+ CategoryInfo : NotSpecified: (:) [Get-App], BlockedDeserializeTypeException
+ FullyQualifiedErrorId : Microsoft.Exchange.Diagnostics.BlockedDeserializeTypeException,Microsoft.Exchange.Management.Extension.GetApp
+ PSComputerName : SERVER
[PS] C:\Windows\system32>Set-App -Identity 7a91e319-a65d-4ceb-909b-12203561dbf5 -UserList testuser -OrganizationApp
[PS] C:\Windows\system32>Get-App -Mailbox testuser
DisplayName Enabled AppVersion
----------- ------- ----------
Cisco Webex-Funktion zum Ansetzen True 43.2.0
WELCOME Development True 1.6.1.0
OTS dms³ False 2017.1.0.0
Meine Vorlagen True 1.0
Abonnement kündigen True 1.0
Aktionselemente True 1.0