"invalid character" and "ajaxcontroltoolkit is undefined" errors on some comptuers

This was an extremely odd error that I'm posting here for others and probably my own future reference.

We recently deployed an Intranet system that used some of the ASP.Net AJAX framework, two users were receiving a javascript error and not displaying a tabcontrol. The first js error was "invalid character at line 0" and the second was "ajaxcontroltoolkit is undefined". This baffled me as all the client machines were identical (in fact they were using a Citrix client so they were definitely identical).

Anyway turns out it's something to do with the last modified date of the assembly if it's somehow set to be in the future compared to the date of the server it is running on (possibly due to a converstion from UK to US date formats for example) you will get this error.

The solution is to simply change the last modified date of the assembly which can be done either using one of several freeware applications available or by using the very simple console application provided at Plain Old Stan's blog. I used this and just set the date back a year and it resolved the issue.

Hopefully that'll be of some use to someone.