I have had to diagnose an issue where Create SharePoint Document Set was failing with the error message "The 'parameters.LeafName' argument is invalid."
The issue occurs when the DocumentSet Name parameter is longer than 128 characters, including any subfolder name (e.g. if you specify "myfolder/mydocumentset" then the entire string is counted in the 128 characters). Special characters that would otherwise be escaped (such as space or ampersand) are included in the count but without escaping (i.e. each special character counts as just one byte rather than the full escape sequence).
This limitation does not make any sense (there should not be a 128 character limit at all), but at the very least it should be documented that this parameter cannot exceed 128 characters, and it would be marginally better Plumsail Actions should fail gracefully with a useful message rather than simply return this nondescript error.