Have you tried accepting your clan invite through the console application?
You may also want to have your clan leader cancel the pending invite, along with any other outstanding invites, then resend your invite.
If you still cannot accept your invite, I would recommend contacting Activision customer support via phone/chat, so they can collect additional information and look into your issue:
So I figured out how to solve this problem. I also couldn't accept the clan invites via the elite website as it always sent me to the connect page.
Tell the clan leader to invite you via the Xbox 360/ PS3 Elite App, then accept that invitation using the Xbox 360/ PS3 Elite App.
The invite via the Xbox 360/ PS3 Elite App is important as there have been cases of invites not showing on the Xbox 360/ PS3 Elite App when sent from a PC.
Another buddy of mine said also said that accepting the invite on an Iphone/ Android App would work. However I have not tested this.
Hope this helps!
On another note I would like to appeal to every rational humanbeing to NOT resubscribe to Elite Premium. Activision released a unfinshed, some may say broken product and their customer support does not seem to care about our concerns. A company should care about their customers. We should not/cannot support this!
Have a nice day!
Once the inviting Clan invites are cleared out, the easiest way to send an invite is to go directly to a members MW3 Playercard, and select "Invite to Clan." If these steps do not work, then this could be the result of those more than cached files.
However, please be aware that @CallofDutyElite tweeted earlier:
"Accepting clan invites is a known issue for some users and we're fixing in the next update."