Changing FTP password sets expiration to 0000-00-00 prevents login
I was attempting to update Wordpress via FTP (since ISPConfig 3.1 doesn't seem to have suPHP on Ubuntu 16.04 with PHP 7.0), and I had to reset the password. It continued to fail to log in.
Syslog continued to say failing to log in, even though I copied the password.
After getting frustrated, I found the config files, found out it pulls directly from dbispconfig and then logged in to that database with phpmyadmin and found that the FTP expiration was not NULL but all 0's.
Setting to NULL in the database allowed login and functioned perfectly.
Changed the password again, it removed the NULL and set back to all 0's.
If date is there, it persists.