Not able to send E-mails to gmail using XOAUTH2

Hi all,

I'm on Debian/Sid for ages. Recently there were almost no package
update because Debian is in full freeze state.

Since 2018 I'm able to receive e-mails via IMAP from imap.gmail.com
using the "gmail" authentication method.

However, I was never able to get the XOAUTH2 method to work with SMTP.
After I've got another reminder from Google that I should disable the
"less secure access" I decided to start another debugging session but
unfortunately I didn't came very far.

If I use the settings STARTTLS/587/XOAUTH2 then I get the following
smtp.log:

  S: 220 smtp.gmail.com ESMTP o16sm8821170wro.63 - gsmtp
  C: EHLO localhost
  S: 250-smtp.gmail.com at your service, [xx.xx.xx.xx]
  S: 250-SIZE 35882577
  S: 250-8BITMIME
  S: 250-STARTTLS
  S: 250-ENHANCEDSTATUSCODES
  S: 250-PIPELINING
  S: 250-CHUNKING
  S: 250 SMTPUTF8
  ... long time without anything happens ...
  S: 451 4.4.2 Timeout - closing connection. o16sm8821170wro.63 - gsmtp
  C: QUIT

I find it interesting that after "S: 250 SMTPUTF8" nothing further happens.
I would expect a line like "C: AUTH XAUTH2 ....."

If I use SSL/TLS / 465 / XOAUTH2 then I only get

  C: QUIT

On "akonadictl restart" I get the following output on the console:

  SASL-XOAUTH2] - Requesting authID![SASL-XOAUTH2] - Requesting token
  ![SASL-XOAUTH2] - filling prompts
  ![SASL-XOAUTH2] - Requesting authID![SASL-XOAUTH2] - Requesting token
  !

Does anybody have an idea how to fix the problem? I would be also willing
to debug this further (including recompiling packages) if I get a sensible
starting point.

Thanks,

Frank