I cannot create a subscription, domain, or mail account. The operation fails with the following error message:
Error: Invalid class string (COM Error 800401F3) at Can't CreateInstance(MEAOPO.PostOffice) (Error code 1)
The mail server registered in Plesk is disabled or has been removed from the server.
Make sure that you are using the supported mail server; check it on the release notes page of the corresponding Plesk version (See the release notes).
Make sure that the mail server is not deleted or disabled; if so, just install the mail server with the autoinstaller: