Ok so ignore the other answer because I had this issue today and it didn't help me at all.
I myself use Singularity and had this error come up multiple times. What I did was install the latest version of Firestorm and when I ran the program it actually runs the process of updating your friends list. After that I logged off and was able to get on my other viewer.
So, get Firestorm, Run it and log in, log out, log in on your personal viewer you use. Done.