Hi Joe,
thx very much for your answer. All the mentioned settings are correct (as they have been stated in other Q&A pairs regarding error 017-714. IP4 only, STARTTLS, appropriate port settings, firmware is updated automatically. The strange thing is that it worked for more than 5 months now with the exact same settings and suddenly stopped working. As already stated, configuring gmail with less secure apps on works, Thunderbird with the exact same settings works and can send email via smtp port 587 STARTTLS and SMTP authorization.