For me, I recently import many MXD files to ArcGIS Pro and one of them I had an issues was that the Data Frame I had may have group with graphics or anything I don't know. Might be better for you to go back to ArcMap and review each layer, graphics that your MXD might have .
Sometimes you might have to fix it once you imported it to Pro. I'd keep back and forth and look what was the cause.
Also, I suggest you to share your screenshot yours to everyone that might help us to give you tips