It looks like non assigned fields in an mtl file, such as "map_Kd ", lead to "String too long" error. Enclosed is an obj+mtl sample demonstrating the issue.
obj filae (no issue with this, just to avoid providing an orphan mtl file)
mtl file that actually causes the error
obj filae (no issue with this, just to avoid providing an orphan mtl file)
mtl file that actually causes the error