What I can see, running the debugger, your file is missing AcDbSymbolTableRecord.
while reading a table there is this code
while (this.chunk.ReadString() != DxfObjectCode.EndTable)
// table entry common codes
while (this.chunk.Code != 100)
Which means that if you don't have the AcDbSymbolTableRecord in your table then this loop will read until the end of the file and then "crash".
the spec from Autodesk says
100 Subclass marker (AcDbSymbolTableRecord)
I don't know enough about DXF to tell you why and where this is needed.But a "simple" solution is to use DraftSight and open the dxf and then save it as a AutoCad 2000 dxf. DraftSight will supply the necessary "tags" and then NetDxf will