Error "Custom Form Cannot Be Opened" in Outlook for Windows

When viewing an Outlook calendar or email invitation message in Quick View, you might receive the following error:

"The custom form cannot be opened. Outlook will use an Outlook form instead. The form required to view this message cannot be displayed."

This usually means the Outlook forms cache is corrupted. To clear the form cache, perform the following steps:

1. Go to File menu >> Options.
2. Click Advanced.
3. Click Custom Forms.
4. Click Manage Forms.
5. Click Clear Cache.
6. Quit and relaunch Outlook.

If the above steps do not work, try the following:

1. Quit Outlook.
2. Delete the cache folder at C:\Users\%username%\AppData\Local\Microsoft\FORMS (where %username% is your login username)
3. Relaunch Outlook.