Here I have exactly the same issue. I have been registered and using v0.13.1 for 3 or 4 days without any problem, but today I can't login ("the given login information is invalid"). Today I don't want to install any mod, I only want to modify one mod installed previously (it is causing me slowdonws), and I can't understand why I must login previously to do that. The program should led me do that being loged out. Also should have the option to remember the password, or even better the option to automatically login when really needed. Regardless it is a great program when it works.
