OK, can't pretend to know the answer to this
BUT I do know that when we release a new code (to fix some glitch or other), you will probably have to log in next time you use MN.
But once you've logged in again, that should be it (until the next release).
Does that sound like what's happening to you? - so, sometimes you have to log in unexpectedly but not all the time.
Or do you find you have to log in several times a day every day?