Weird problem: Activation/forgot password links sent in emails don't work

[KTT Robot]

Activation and forgotten password links, which are sent to users via email, do not work when clicked from email accounts.

When clicking from Gmail it just goes straight to the forum index, instead of the activate page. HOWEVER, clicking the address bar, and then pressing enter to load the URL (without a referer?) will make it work as it should. I am also getting reports of this problem from non-gmail users, but I don't have any other webmail services to test.

Can anyone shed some light on this issue, please?

[SlammedDime]

Do you have any SEF url mods like Pretty Urls or SimpleSEF installed?  What mods do you have installed?  Link to your site?

[KTT Robot]

Do you have any SEF url mods like Pretty Urls or SimpleSEF installed?  What mods do you have installed?  Link to your site?

kanyetothe.com/forum

No mods except "stop spammer", but no change has been made to mods lately, and this is a recent issue.

[KTT Robot]

For example, this URL: http://www.kanyetothe.com/forum/index.php?action=activate;u=1234567;code=blah [nofollow]

If you were to paste it into your address bar it would work fine (although it would give an error that the user doesn't exist, as it should).

However, if you were to email it to yourself and click from there, it would just show the index page.