Macintosh Viewstate Errors

[Dave]

The public site I maintain is gathering the following 500 error:

"The viewstate is invalid for this page and might be corrupted"

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, but no reply.

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.

Any ideas or suggestions would be most welcome.

Dave