On attempting to update any of the mailbox storage quotas through the web interface to zero, the operation reports success on screen but the setting is not made.
On checking the event log on the ARS server, the following message is logged:
Pre-processing operation on object caused a policy violation Policy: AzureUser Object: CN=CN_NAME,OU=OU_NAME,OU=OU_NAME,DC=DC_NAME,DC=DC_NAME,DC=com
Details: Administrative Policy returned an error. One or more errors occurred.
The ARS console shows the value correctly:
Waiting for fix in a future release of Active Roles Server.