Does anybody know, why Java Games are not situated in the center of the screen and why there are some Graphical Bugs on the right?
How can I improve it?

Probably those Java games were not written using Nokia's Series 60 Java SDK (Software Development Kit) and, thus, do not use Nokia's Series 60 Java UI libraries.

Try, e.g., Gameloft's Java games specifically written for Series 60 phones like your 3660 & you'll see the difference.

thanks