Okay this is quite a specific oddity and will only be of use to anyone inside the UK, and even then only if you’re connected to the academic ja.net network and using Internet Explorer to make video conferencing bookings.
If you’ve got anyone struggling to make a new video conferencing booking after logging in- more specifically when you try to, it says it doesn’t know who’s logged in and the only option is to try loggin in again (this gets into an infinite loop if you let it!)- try checking the bit-version of IE you’re running. It might simply be a case of switching from IE x64 to IE x86- I noticed this morning that the jvcs site worked fine on my client- both through Opera and IE- and so when I check ed on the “faulty” machine I noticed it running IE x64 by default. Switching to IE x86 seems to have resolved the problem.