No, there is no expiration date on the credentials. But there could be on the either of the systems.
Basically, you have to choose between security and simplicity. I used a strong password on both systems, set them to never expire. The username and password are the same on both systems (not because they have to be, just because it's less confusing), and I dont have a problem with the credentials being remembered.
If you set passwords to expire, the credentials will not work when the password changes, obviously.
Passwords have not expired, and you have not done a system restore or something else to roll back settings, and persistence is Enterprise, and the account is not disabled or locked, you have not disconnected/reconnected the drive?
I searched google, it seems to be a common problem, but the answer was almost always the credential vault. I don't know much about OSX, but I do have a Linux system, and this works fine with that system.