When a Voicemail message is opened in Microsoft Outlook in ViewMail for Outlook (VMO) form, the The Custom form could not be opened. Outlook will use an Outlook form instead error message appears.
This issue carries over to other PCs, but does not impact the functionality of the PC itself.
This issue is tracked in Cisco bug ID CSCae06812. One of these methods usually resolves this issue:
Refer to Cisco Unity VMO Error: "The custom form could not be opened." for more information on how to complete these tasks.
Cannot start, stop, schedule, reset, power up, power down, run, initialize, open, close, activate, deactivate or load