Unable to update mail user password with encrypted value in case of maibox user has access to Control Panel.
The issue is considered to be a software issue #PPA-2287 and will be fixed in future product updates.
Please use provided ID to track maintanance in PPA release notes.
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As temporary workaround, you can restrict access of mailbox user to CP:
Login to Customer CP.
Go to Mail > Mailbox.
- Uncheck option "Access to the Control Panel"**.