Go BD87 and enter the number of your failed IDoc, execute
Select the IDOC and choose from menu EDIT > Restrict and process.
The next screen is a selection screen for manual processing, prefilled with the earlier entered number, here remove the flag for background processing at the bottom.
Then process your selection until you get to a screen with an individual IDOC number (the screen is called Status display) Here choose from menu Edit > process > in foreground (or foreground from error)
Then you should see at which screen and field SAP is when it fails.