When changing a user's cPanel password, the password does not seem to take, and you are asked to reset the password again on the next login.
This can happen when the account is over quota. To confirm, check the cPanel error log at
/usr/local/cpanel/logs/error_log for entries for your user.
This will look like this:
The system failed to create the file "/home/$user/.cpanel/email_accounts.json.lock"
(as EUID: 1234, EGID: 1234 1234) because of the following error: Disk quota exceeded
You can modify the quota for the user via WHM > Account Functions > Quota Modification.