Can you post an example email that triggers this error ? You can try registering with similar characters (shuffle them if you don't want to share your actual address) at the demo forum to see if it triggers an error there as well.
I suspect you have some kind of special characters in the email address.
Flarum default email validation uses Laravel email validator, which uses PHP FILTER_VALIDATE_EMAIL
filter, which is described as:
Validates whether the value is a valid e-mail address.
In general, this validates e-mail addresses against the syntax in RFC 822, with the exceptions that comments and whitespace folding and dotless domain names are not supported.
I had a look at the spec but I'm not really sure which characters are allowed https://tools.ietf.org/html/rfc822#section-6.1 . It would be strange for gmail to allow emails that don't validate in all clients though 🤔