Make sure you have the latest
JDBC driver jar (and only that jar) on your system. An old driver can be attempting to use the original connection protocol that the new server will reject.
Did you do your
testing from your web server using a JDBC based tool? There might be security issues, e.g. MySQL user id not authorized from that machine; a firewall setting is not correct; and the like.
Don't depend on the MySQL GUI admin tool since that actually uses a slightly different connection protocol than JDBC.
Did you just log in or did you login in and attempt to use the jforum database/tables (if any)? It could be the id can connect but can't access the database (catalog) you are using with jForum.
Well, that's my top of mind list of possible problems.
[originally posted on jforum.net by monroe]