please consider using the english version of error messages in the future to make it easier for others to help you.
It seems that in the 7.1.x versions the handling of emails might have changed. You do not see error messages because they are not being logged (at least not in PE 7.1.6). You can try the code changes I suggested in Invitation problem [Seafile Pro 7.1.7] to actually see the SMTP error codes - please remember to adapt them accordingly if the file has changed in PE 7.1.8. Most likely the invitation mails are being bounced back for some reason. Since Seahub uses a synchronous transfer at the moment, those mails are not being queued for any sort of retry and the user receives the error message you posted.
We used a local postfix installation on all Seafile/Seahub servers to enable queueing and resending all emails - this might also be a possible workaround for you. An added bonus is that postfix can actually show statistics for bounced mails etc. from the external SMTP relay that is used.