Error in JIT user provisioning

Questions
1

Hi,
during the first login through Microsoft IdP of some users we have this error: “Unable to JIT user from the Identity Provider”

Searching in the system logs we found that “preferredLanguage field failed validation with value ‘0x0409’: For property ‘{0}’, string value of {1} is not a valid language priority list from RFC 7231 Section 5.3.5.”

What does it means? There is something wrong on the user configuration on the Microsoft side?

Thanks to anyone who can help

2

Does the value on the Microsoft side have an underscore(_)?
https://support.okta.com/help/s/article/AD-Import-Error-preferredLanguage-is-not-a-valid-language?language=en_US