We had a similar issue recently whereby users were being prompted for login credentials if we used the "Automatically detect settings" option in IE that resulted in using a wpad.dat file.
After thorough investigation we found that PAC/WPAD files are
case sensitive. Somehow, somewhere and at sometime the EV client appears to make contact with the EV server and the URL information can be sent or received in either upper or lower case. (Symantec Support confirmed this)
I am not 100% sure when the failure starts, but we found by adding 2 lines to the WPAD file to convert all URL references to lowercase that the issue was resolved.
The two lines to adde at the start of the WPAD file are:
var lhost = host.toLowerCase();
host = lhost;
The background can be found in this very handy website:
http://www.returnproxy.com/proxypac/index.php?option=com_content&view=article&id=54&Itemid=83
or by going to
www.returnproxy.com/proxypac and then clicking on the "
PAC File Tricks and Tips" link.
Hopefully this helps someone else as it took us several days and lots of hours to find a cause and solution for this issue.