I keep getting the message "Something went wrong while displaying documents" in AutoFill mode

Despite the fact that you may not use iCloud or iCloud drive this error appears to be strangely related to that setting on your Apple ID. This error message seem to happen when you have your “iCloud Drive” disabled for your Apple ID on your device. To check this go to:

iPhone or iPad Main Settings > “Your Apple ID at top of screen” > iCloud > iCloud Drive

If you have this switched off, turning it on (a device restart may be necessary) usually solves this issue.