Cause: Your E28 is running the older-style spindle dust caps.
Solution: Purchase the newer-style "concave" dust cap!
Part number: 31206777789
The part is the same between left and right.
They're extremely inexpensive, as they cost about $2-$5 ea depending on where you get them. I'd give our endorsed vendors a shot first, though.
Simply pop your old ones out, and put these in. Give a nice whack with a rubber mallet and you're set!