Hila!
I have finally figured out a solution to the problem of a generic error when trying to import C300 MXF files. If you do not have Final Cut Pro installed on your computer, you are simply missing the MXF import codec. You can download it along with the ProRes codecs by following the instructions here: https://www.martin-thoburn.com/vj/install-apple-prores-codec-without-final-cut/
Hope this helps!
Colin