had same effect here on my sites since some days but SOLVED on my side.
Problem was that my Firefox has kicked out my special Proxy-Settings and as I had defined a IPs in my "White-List" in TP-Admin area I was unable to connect because my provider has changed the whole dynamic IP range which wasn't activated to connect at TP
@ ip0li : should be nice if there will be posted any error message on the login-screen like "not allowed to access" or "wrong password" or anything like this that it is better to know where the problem exist