SUMMARY
XOAuth authentication (using for example by KMail to access Gmail accounts) does not work properly by default on User Edition due to a problem in SASL_PATH. See related bug https://bugs.kde.org/show_bug.cgi?id=505428


STEPS TO REPRODUCE
1. In KMail/Akonadi, add a new IMAP account with XOAuth authentication.
2. OAuth login screen is shown properly in the browser.
3. The IMAP server cannot be reached, and email is not accessed.

OBSERVED RESULT

The created account does not work, and the following message appears in the logs:

org.kde.pim.kimap: sasl_client_start failed with: -4 "SASL(-4): no mechanism available: No worthy mechs found"


EXPECTED RESULT

Folders and emails from the created IMAP account should appear in KMail.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
(available in the Info Center app, or by running `kinfo` in a terminal window)
Linux/KDE Plasma: KDE Neon 6.3
KDE Plasma Version: 6.3.5
KDE Frameworks Version: 6.14.0
Qt Version: 6.9.0

ADDITIONAL INFORMATION

I managed to solve the problem by following these steps:

1. Create a new script in ~/.config/plasma-workspace/env named 'sasl_path.sh' with the following content.
  export SASL_PATH=/usr/lib/x86_64-linux-gnu/sasl2

2. Create a new symlink in /usr/lib/x86_64-linux-gnu/sasl2 since libkdexoauth2.so didn't exist:
  cd /usr/lib/x86_64-linux-gnu/sasl2
  sudo ln -s libkdexoauth2.so.3 libkdexoauth2.so