first of all, did you install any new plugins prior to when this started happening?
also, did you try both -
1. putting your password in,
2. just pressing ok without putting in the password. or does it sign you out each time regardless if you do any of these things?
if it does, you can try a couple of things:
try running msn without plus!, if it lets you sign in, than i suggest you -> uninstall plus along with the prefferences, and reinstall it.
if msn signs you out even when plus is not running, try a clean install of msn (uninstall msn and than install it again).
most importantly, which versions of msn and plus! are you using?
check here for the newest versions. if you are not using the lastest ones - do an update:
http://messenger.msn.com/download/
http://www.msgplus.net/download.php
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