Process the IDOC in foreground.
Go BD87 and find your failed IDoc.
Select the IDOC and choose from menu EDIT > Restrict and process.
On the next screen (is a selection screen for manual processing, usually already prefilled with the earlier selected number) you have to remove the flag from background processing at the bottom.
then you process the selection until you get to a screen with an individual IDOC number (screen 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.