Sorry, I missunderstood something. I believed that you also administrate the Drugbank site.
We have checked the Drugbank site. Applet loads fine for us on that site. As I see, Drugbank site uses Marvin 5.1.1. The expiration date of the certification in this release is 2010. So the validity of the license can not be the problem.
I assume that rejecting trusted certificate can cause the problem. I recommend to accept ChemAxon trusted certificate permanently when applet asks it again.
If it does not help, it means that your GCJ Java plugin can not handle properly signed applets.
In this case, removing GCJ plugin and install Sun distributed Java plugin can be the solution.
We are continously testing Marvin applets under Linux with Sun Java and it works fine.
Anyway, we highly recommend to use Sun distributed Java since we can provide support for only this Java distro.
http://www.chemaxon.com/marvin/help/applications/install.html#requirements