Okay, I spent quite a bit of time on this and I have finally found a fix to the "Sorry - there were problems logging you on." issue. Firstly, it is NOT It is not an authentication issue.
Seems it recognises the username and password OK. Seems kplaylist cannot generate a session cookie because it cannot write the session info to the database.
Cause
Turns out this is a MySQL database-related issue. New default settings in MySQL 5.7.22, set its the "sql_mode" to:- "ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION"
Solution
Change MySQL 5.7 default settings in /etc/mycnf (or wherever your my.cnf file is) by adding the following, and restart mysql...
[mysqld]
sql_mode = NO_ENGINE_SUBSTITUTION
By specifying "NO_ENGINE_SUBSTITUTION" it defeats all the other sql_mode restrictions and KPlaylist plays nicely again! Strictly speaking, the login PHP code for KPlayist really could do with some revising so this workaround becomes unnecessary. But for the time being, this workaround seems to resolve the problem.
I hope this helps someone. :-)