"Unexpected error". Locked out of site, but only on one computer!

As I have no idea what causes this I'm clueless as to what to search for (besides the error log and I cant find anything that way) so please accept my apologies if this is a dupe.

I change user via Devel module on one browser window (whilst logged in on several FF browser windows).
Sometimes I get completely locked out of the site (not always).

I get the standard error message on screen and the error log reports:
"Recoverable fatal error: Argument 2 passed to drupal_array_get_nested_value() must be an array, null given, called in /home1/public_html/sites/all/modules/conditional_fields/conditional_fields.module on line 443 and defined in drupal_array_get_nested_value() (line 6368 of /home1/public_html/includes/common.inc"

Strange thing is, I cannot log in from any browser type (IE, Opera, Chrome) on that machine. Even after I've cleared all caches. However, when I use another machine (same IP) no problem.
I disable the Conditional Fields module from that machine and everything works fine again on the original machine.