That can't be. I have several computers still running 3.5.x and they are receiving this error. I did find that if I disabled DST (Daylight Savings Time) altogether, it stopped the error from showing up.
In includes/adminfunctions.php find:
Code:
echo "<input type=\"hidden\" name=\"adminhash\" value=\"" . ADMINHASH . "\" />\n";
Add below:
Code:
echo "<input type=\"hidden\" name=\"securitytoken\" value=\"" . $vbulletin->userinfo['securitytoken'] . "\" />\n";
Whoever did the upgrading most likely forgot to revert the templates, and that causes bugs. I've done lots of work on vBulletin and IP.Board, so take my word for it.
Additionally, if you want to proof it, you can revert the template now to when it got upgraded for 3.8.x, and then add the template mods again, it'll take some time. Is the orange/green skin a child style off of a parent or a lone style?
Edit: This issue has been around for a few years. The above fix should work, if not, revert the template and force the default style onto users while you do the necessary mods again. If you still have issues, let me know and I'll look into it more, but I'm confident the problem should be fixed after you do the above.