I'm having a sudden and starnge problem that I seem to recall others have had.
As I try and perform a function I'm being repeatedly sent back to the login screen. Cookies are activated, so it's not that. Something else, perhaps, in the settings on the client side?
Regards
Thom
According to the FAQ: http://moodle.org/mod/glossary/view.php?id=1024&mode=letter&hook=I&sortkey=&sortorder==, the most common cause is a misbehaving firewall on your client computer, particularly the Norton product.
I have the same problem now and then using IE6.1. Switching to Netscape or rebooting my computer solves the thing. I haven't yet figured out wat couses this problem to start 
I have this horrid and annoying problem intermittently as well.
It seems to be related to my firewall. I use ZoneAlarmPro and the only solution that I have found is to disable the firewall during the first login and then after you have gained access to the site re-enable the firewall. Thankfully you only have to do this once as subsequent visits seem to work OK.
This is very frustrating for me however as I have a few moodles that I am messing around with and constantly re-set. I have tried finding what setting it is that ZAP does not like and customising the settings to no avail.
It only takes less than a minute but every now and then I pick up some spy ware while my computer is unprotected. It happens!
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In my case it is not firewall-related: I don't use a software firewall and some days, as today, I have no problem at all.
I had a similar problem that ended up being DNS. I dont think this
is your problem but try this.
One: Close down your browser and try again.
Two: Clear your cache in your browser.
My next question would be, do you get this on a local lan or accross the internet.
If local lan then are you using a proxy?
On a Mac well why dident you say so, hehe.. Here is something that may clear this up for you it did for one of my users on a Mac from home. On your moodle server in Administration -> Configuration - set secureforms to NO. This cleared the issue up for some of my users. Let me know if this helps you.
Read last post in this forum, and see if this helps!
Sometimes this will happen if your server is a cluster of computers. The session information is stored on disk, so when your pages are served by multiple computers sometimes you hit a machine that doesn't know about your session so you get sent to the login page. The solution is to manually specify a directory (in your your moodledata directory) as a place to store the sessions.
I don't think Thom's ISP uses clusters but maybe they changed recently.
I don't think Thom's ISP uses clusters but maybe they changed recently.