Viewstate Error and Macintoshes?

The public site I maintain is gathering 500 errors of the dreaded “The viewstate is invalid for this page and might be corrupted” type. Strangely enough, the only users seeing this error are Macintosh users, using IE! The problem is not isloated to a single page, it seems to be happening on any page, but not necessarily every page for that user. In some cases it is clear that the user was getting around fine (they were logged in as members to the site), and later the problem occurred.

I cannot find anything on the web except a solitary Usenet post of a person describing the exact same problem.

Here are the agent strings of the browsers recorded when the error occurred:

Mozilla/4.0 (compatible; MSIE 5.0; Mac_PowerPC)
Mozilla/4.0 (compatible; MSIE 5.17; Mac_PowerPC)
Mozilla/4.0 (compatible; MSIE 5.15; Mac_PowerPC)
Mozilla/4.0 (compatible; MSIE 5.16; Mac_PowerPC)

According to the log files, MacPPC users are just under 1% of the site users, so this problem is clearly not a showstopper, but I hate for any users to have a problem. I am hoping to get a Mac set up to test the problem locally, but the agent string doesn’t give me any Mac OS versions….