I just wanted to post this somewhere, maybe this helps somebody in the future ...
I'm using W3TC together with "WordPress Newsletter Plugin" from Tribulant. I have database cache enabled for not-logged-in users.
I had the problem that when a user registered to my page and sign's up for the newsletter he get's an invalid authkey per email and so can't process the double-opt-in.
The problem is that the process of generating an authkey is run multiple times and the function checks if there is already an authkey in the database. As this query is cached by W3TC it always returns the same result. So the function always incorrectly generates a new authkey, overriding the existing and so making it invalid.
This problem can be solved through addding "wp_wpmlsubscriberslists" to the ignored query stems in the database cache settings.