I also get that error sometimes and it's happened for a couple of months on my end. I'm not sure what causes it, perhaps increased server load. I'm pretty sure that tech is aware of the issue, as they have notified us a few times they were working on performance issues. Hopefully it will be over soon!
I'm also having trouble over the last week. Seeing some other comments about this elsewhere so wonder if it's 3fc or further up their system but not overall. Today I'm fine but am from a different computer. Could it be my internet provider?