A few times in the past I've got the error report:
assembling FileName.Src... FileName.Src: error: A3906U: Malformed via file '.\obj\FileName._ia'.
I can't find any reference to this error code.
So far, the only fix I've found for it is to remove the file from the project and then re-add it. No change to the source module itself.
This is with the latest release of the MDK - But I've seen it occur with earlier ones.