Microsoft Entra Connect sync stopped, request upgrade and library not found

Thanks Kidd_Ip​ for your suggestion; however, during the search to understand what's happening, I found something (for me) crazy.

I updated to the version 2.5.79.0 (last available from Entra Connect Sync blade for me) in march and until now all is working; I tried a sync, obtained an error in cloud connector during the export (stopped-extension-dll-exception) and after this I searched for in Windows Logs Application event I found that warning:

Assembly version ("2.4.131.0") in configuration of ECMA2 connector: "imncloud.onmicrosoft.com - AAD" is earlier than the actual version ("2.5.79.0") of "C:\Program Files\Microsoft Azure AD Sync\Extensions\Microsoft.Azure.ActiveDirectory.Connector.dll".

Found also errors, bound to the file not found with text like this (event ID 106):

Failed to connect to Windows Azure Active Directory during export. Exception: System.Management.Automation.CmdletInvocationException: Could not load file or assembly 'file:///C:\Program Files\Microsoft Azure AD Sync\Bin\Microsoft.IdentityModel.Clients.ActiveDirectory.dll' or one of its dependencies. The system cannot find the file specified. ---> System.IO.FileNotFoundException: Could not load file or assembly 'file:///C:\Program Files\Microsoft Azure AD Sync\Bin\Microsoft.IdentityModel.Clients.ActiveDirectory.dll' or one of its dependencies.

or this (event ID 6801):

The extensible extension returned an unsupported error.
 The stack trace is:
 
 "System.Management.Automation.CmdletInvocationException: Could not load file or assembly 'file:///C:\Program Files\Microsoft Azure AD Sync\Bin\Microsoft.IdentityModel.Clients.ActiveDirectory.dll' or one of its dependencies. The system cannot find the file specified. ---> System.IO.FileNotFoundException: Could not load file or assembly 'file:///C:\Program Files\Microsoft Azure AD Sync\Bin\Microsoft.IdentityModel.Clients.ActiveDirectory.dll' or one of its dependencies. The system cannot find the file specified.

This is a screenshot:

I have done another control: in Entra site, under Entra ID->Entra Connect->Connect Sync, I clicked on Click here to use the tool in Is Cloud Sync the right solution for you? that loaded the page Guide to add or sync users with Entra Connect or Entra Cloud Sync then I clicked on Troubleshooting and a Microsoft Entra Cloud Sync queries problems is found 

click on Next and the result is:

The version recognized is 2.4.131.0, the last before the upgrade; I don't know why the system, at a point of time, recognized this version and not the last installed (in control panel the version installed is 2.5.79.0.

I also found in a trace log that you suggested to check the message:

Version 1.5.3599.0 is below the required version 1.5.4326.0 of product Microsoft Azure AD Connect Agent Updater is installed and requires upgrade

and under event Windows Logs->System an error on distributed COM (event 10016): I see the error every day, one time a day, but I notice that the description of the error is this before the stop

The application-specific permission settings do not grant Local Activation permission for the COM Server application with CLSID 
{D63B10C5-BB46-4990-A94F-E40B9D520160}
 and APPID 
{9CA88EE3-ACB7-47C8-AFC4-AB702511C276}
 to the user [domain]\Administrator SID (..........) from address LocalHost (Using LRPC) running in the application container Unavailable SID (Unavailable). This security permission can be modified using the Component Services administrative tool.

while immediately after the stop

The application-specific permission settings do not grant Local Activation permission for the COM Server application with CLSID 
{D63B10C5-BB46-4990-A94F-E40B9D520160}
 and APPID 
{9CA88EE3-ACB7-47C8-AFC4-AB702511C276}
 to the user [domain]\ADSyncMSAc2ec7$ SID (......) from address LocalHost (Using LRPC) running in the application container Unavailable SID (Unavailable). This security permission can be modified using the Component Services administrative tool.

the user is changed from Administrator to ADSync....

I don't know what to do; a quick search on chatgpt suggest to reinstall but I followed the steps on my test tenant and when I reinstall the Connect Sync it doesn't find the previous configuration (chatgpt suggested that uninstall the Connect Sync doesn't cancel the local db, but it seems not true).

I have no backup of the configuration, or better, I have the json file that I have done as backup before install in march (and is quite good because in the meantime I add in the scope only another OU); I can't do the backup because the PowerShell cmdlets don't work and via UI is impossible.

Do you have other suggestion? Maybe I can save the local db, but how? Any suggestion is appreciated. 

Please try this:

1. Check for Inconsistent Installer Packages

• Open %ProgramData%\AADConnect\ and inspect the latest installation trace log.
• Look for entries like GetInstalledPackagesByUpgradeCode to identify if older versions are still registered.
• If the wizard detects remnants of older installations, it may trigger the upgrade button unnecessarily.

2. Manually Remove Corrupt Install Entries

• Use msiexec /x {ProductCode} to uninstall any ghosted or partially removed packages.
• Be cautious, do not edit the registry directly unless guided by Microsoft support.

3. Reinstall Microsoft Entra Connect Cleanly

• Download the latest version from the  official Microsoft Entra Connect page.
• Uninstall the current version completely.
• Reboot the server.
• Install the new version and reconfigure sync.

4. Verify .NET and Dependencies

• Ensure that the required .NET Framework version is installed and healthy.
• Use sfc /scannow to check for system file corruption.

5. Check for DLL in NuGet or SDK

• The missing DLL (Microsoft.IdentityModel.Clients.ActiveDirectory.dll) is part of the Azure AD Authentication Library (ADAL).
• You can manually download it from  NuGet and place it in the expected path, but this is a workaround and not officially supported.