Yeah I think that will occasionally happen if the RTB server is experiencing high CPU usage which has been happening recently due to another user on our shared server running some CPU-intensive applications (this has now been dealt with, by the way) - I have removed the popup from displaying in the next RTB and down-graded it to a console error, however this shouldn't be happening repeatedly unless you actually have entered details incorrectly since the system will try to auth you up to 10 times then fail. Sessions are then established between you and the server so the process should only ever be done once unless your session expires (but you'd have to be playing blockland for a few days straight for that to happen).
Anyway, i'll be looking into whats going on there later.
@Marcem, this is an RTB error not the Auth Fail blockland error that closes your internet server. The RTB one just has a popup message.