When logging in the user is being recognised okay, as it says invalid user if not.
It definitely seems to be a password issue, and it seems to be a 24 hour thing, not overnight as such.
I tested this as I had a test user in there I set up early PM and that worked fine the next morning, but then was locked out post their original sign-up time.
If (when logged in as Admin) I change a user password then that user (who previously couldn't get in) can then login again.
(I'm not sure if that's true if a user tries to update their own password - I've set up some users to try that with tomorrow).
However, it looks like 24hrs later that password has been affected again.
So I don't think that it can be post data being incepted, but does seem to be something affecting the passwords 24hrs after they are set up.
The only accounts that aren't being affected are the main admin and paid s2member accounts.
Where is the EOT information stored against users? I ask, as I've added an EOT date/time for the Editor who has been affected to see if that makes any difference to their login situation. I thought the EOT data was being pulled from PayPal but as it let me enter something in there, and it's been saved, I'm assuming it must be being saved somewhere locally?
I'm not getting this issue on the dev site, and I've double checked if the settings are the same, and they now seem to be.
It could indeed be a plugin thing (there are a couple of differences), or something to do with the host, or possibly hacking (but I've not read anything about this being an issue, i.e. this type of hack)?
My thought is that the passwords set are being automatically altered after 24hrs, but I can't see what could be doing that as selectively as this (i.e. free members and editors and not paid members or admins).
(I'll need to check with the site owner re: allowing you access directly - as it's their PayPal account etc - Thanks for the offer, and I'll let you know as soon as they make a decision... not always quick!)
I'm tearing my hair out