Today we're dealing with a problem executing an XPage that uses a Java class, developed inside Domino Designer.

The user is getting an HTML Error 403 on clicking a button from the XPage that instances the Java class. No other information can be obtained from the server console or from \data\domino\workspace\logs directory

The reason for this problem is the lack of permissions for the Java class to execute in the server.

Solution
It's necessary to modify the security restrictions for Java excution in the
/domino/jvm/lib/security/java.policy
file

You can add a EVERYTHING CAN BE DONE ENTRY
grant { permission java.security.AllPermission; };

or a more restrictive, per database, entry

grant codeBase "xspnsf://server:0/mypathdbinlowercase.nsf/-" {
     permission java.security.AllPermission;
};

There is an interesting debate at http://www.wissel.net/blog/d6plinks/SHWL-8JYAT5