Hey all.
There were some authentication issues through Steam yesterday that should now be resolved. Sorry about that!
If you ever get prompted to manually sign in when you wouldn't normally, try a restart of your Steam client. Ultimately though, it'll likely be an outage between ourselves and Steam and not something you can brute force. Always avoid drastic actions like reinstalling, and either wait it out or drop us a line in support to find out what's up : )
-Luna