First of all check which version of Java you have. Latest Marvin requires at least Java 1.5. See details here:
Which Java versions are supported by Marvin?
If you have not got an earlier Java version, please update to a newer one.
If your software environment is proper, clean Java cache, probabably some deprecated resources were rested in the Java cache.
How I can clear JRE cache?
If the problem is still existing (after clearing of the cache), the error may be on your server. Certain jars can be corrupt there. Unwrap the marvin applet package into an empty directory on the server and refers you applet directory to this pure package.