There may be two reasons:
(1) You browser may have stored an invalid login-cookie. Please try if you encounter the same issue in another browser - if it works there, the cookie is like the cause. Clear all cookies from SoSci Survey.
(2) You may be connected over an Internet connection that rotates the IP address. If that is the case, you should see a error message about the IP mismatch. You may disable the IP check (which is a nice but not crucial security feature) in the account settings.