Can't send mails anymore

I’m using InvoiceNinja v 5.4.9 and when trying to send an invoice or quote I get 500: Server Error.

The corresponding error on the server looks like this:

AH01071: Got error ‘PHP message: PHP Fatal error: Declaration of Egulias\EmailValidator\Validation\MessageIDValidation::isValid(string $email, Egulias\EmailValidator\EmailLexer $emailLexer): bool must be compatible with Egulias\EmailValidator\Validation\EmailValidation::isValid($email, Egulias\EmailValidator\EmailLexer $emailLexer) in /home/users/ahm/www/invoiceninja/vendor/egulias/email-validator/src/Validation/MessageIDValidation.php on line 23’

And the error in larval.log is this:

production.ERROR: Declaration of Egulias\EmailValidator\Validation\MessageIDValidation::isValid(string $email, Egulias\EmailValidator\EmailLexer $emailLexer): bool must be compatible with Egulias\EmailValidator\Validation\EmailValidation::isValid($email, Egulias\EmailValidator\EmailLexer $emailLexer) {“userId”:1,“exception”:"[object] (Symfony\Component\ErrorHandler\Error\FatalError(code: 0): Declaration of Egulias\EmailValidator\Validation\MessageIDValidation::isValid(string $email, Egulias\EmailValidator\EmailLexer $emailLexer): bool must be compatible with Egulias\EmailValidator\Validation\EmailValidation::isValid($email, Egulias\EmailValidator\EmailLexer $emailLexer) at /home/users/ahm/www/invoiceninja/vendor/egulias/email-validator/src/Validation/MessageIDValidation.php:23)
[stacktrace]

Because I don’t know what to do else, I tried:

php aritsan migrate
php artisan optimize

Any further advice for me?

Seems to be fixed in v5.5.7.