Error when use Opt-in plugin in wordpress: Failed to parse XML-RPC request: Invalid byte 1 of 1-byte UTF-8 sequence


I test my Opt-in form (plugin Infusionsoft Official Opt-in Forms in Wordpress), I fill with the Vietnamese characters, but it cannot admit data and show this erro " Failed to parse XML-RPC request: Invalid byte 1 of 1-byte UTF-8 sequence.".
Please help me!
Thanks so much!

Yeah, vientamese characters won’t carry over without issues because the collation (internationalization) doesn’t exist for them from what I understand.