When attempting to work with content in a custom culture the following error message may be thrown. This is caused to the way that the Ektron CMS would create culture information based on an existing language.
Inner Exception Type: System.Globalization.CultureNotFoundException
Inner Exception: Culture is not supported. Parameter name: culture 1048577 (0x100001) is an invalid culture identifier.
Inner Source: mscorlib
This has been officially parched in the latest cumulative site update for version 9.1 sp3.