Hi
Do you have the system logging level in verbose logging mode? If so, check if there are any errors related to SSL.
If not, enable the version logging form under the management console > Dashboard > Activity log > settings, restart only the 3cx system server service, and re-check the system service log.
If no SSL errors can be seen, re-check your app permissions match those in the screenshot below.
After ensuring that the graph permissions are granted correctly, as above, then navigate back to the management console > Settings > M365 integration section > Step 3 and press the sign in the Microsft 365 and sign in with the 365 global admin account, and press Accept, then check if can change the settings under the contacts or user sync sections.
If you have any SSL errors in the logs, then kindly follow the steps below and let us know if the issue is still there:
• Create a full backup of the PBX and keep it in a safe location for security reasons.
• Connect via an SSH terminal or a file tool such as WinSCP, and search for the following.
/var/lib/3cxpbx/.dotnet/corefx/cryptography/x509stores/ca
• Delete or move any .pfx certificates that are found under this location.
• Then restart the 3cx system server service, check the O365 integration, and let us know.
