Hi @timcastelijn,
Rhino does not provide much in the way of end-user tools for diagnosing file corruption issues.
There are the Audit3dmFile and Rescue3dmFile commands. The latter isn’t really an end-user tool.
In many cases, you’re better off just submitting the corrupt file so one of our developers can investigate.
With your file, I can see Rhino is trying to read the surface parameter mapping tag, or ON_MappingTag, which is part of the mesh data structure. The “chunk” of memory Rhino is trying to read is of the incorrect type, thus the error.
– Dale