I have a wiered problem with my org. This came up after i enabled Person Account. Now i don't want to use it but as we cannot disable it i am missing some of basic functionality.

The most basic one is Lookup Fields on Opportunity are not getting saved. Here are the screen shots have a look. I have tried looking into before Trigger, after Trigger, in Workflow, and Process Builders, but not found any clue to my bad luck. Can anyone help with this?