Jonathan,
After upgrading, have you recustomized your custom Javascript/HTMPL forms?
Does the problem happen if you were to rename NX_ROOT/site/mods/www directory to something else (www_orig), open a command prompt and type: pdm_webcache.
Close out your existing browser and then retry your use case now
The error seems to be around some custom code that might be interfering with 14.1 code.
_R