Windows update log filesare present under $Windows.~BT\Sources\Panther. There are two files setupact.log and setuperr.log.

Additionally, during upgrade setupdiag automatically creates logs under %windir%\logs\SetupDiag\SetupDiagResults.xml.

Setupdiag is present under %SystemDrive%$Windows.~bt\Sources

This above file will be useful as it has the appropriate message.

Check at what stage the error occurred, and what is the error code, which can be checked against Win32_Error codes.

For example:

0x00000070 - ERROR_DISK_FULL

How to run setupdiag if not present

SetupDiag.exe /Output:C:\Temp\Results.log