Hi Jon,
It's unfortunately a pretty generic error. I could throw some questions out there, though, and maybe it will inspire another person to have other thoughts.
Does the bibid look valid to you? It should be the full bib id without the check digit. I am pretty sure the code operates under the assumption that all bibids are the same length in all installations of III. If that's not actually true, it would cause problems.
Can you get to the details page in the non-js version? you can append '?forcejs=0' to your mobilecat url to check this.
Best,
Anna