Password Retention in Preferences
I find that the password for syncing is not retained in the preferences menu. If this is dependent on a cookie it shouldn't be as the actual site conditions for accessing the database may vary a lot.
This is an old discussion that has not been active in a long time. Instead of commenting here, you should start a new discussion. If you think the content of this discussion is still relevant, you can link to it from your new discussion.
Upgrade Storage
Also note the known issue of password changes for WebDAV syncing:
http://www.zotero.org/support/1.5_beta_known_issues