Advanced vCenter settings
VirtualCenter.VimPasswordExpirationI... (VIM Password Expiration). Do a filter search on 'password' to find it.
Change this value to 3 or something so you won't be reminded it defaults to 30 days, this should fix it.
Yes I know this post is old.. but I searched for this myself.. and I finally found the solution (I think) because it was annoying me.. since I recently upgraded to vCenter 6.5
