After resetting a user’s password, if it immediately expires the most likely cause is that PT-X drive is using this account to connect to PT-X.
If you are using PT-X drive please see this article.
If you are not using PT-X drive, you should check that the web address you are using to access PT-X is https://uk.pt-x.com and that you are not using an old link from a password reset email. One other thing to do is to clear your browsing history.
If you are please contact Bottomline support on +44 (0) 118 925 8250 and choose option 2 for technical support for this to be investigated further.