The sandbox applet is signed with valid cert and it is trying to read "java.test.blah". After accepting the signed sandbox security dialog there is another blocked dialog coming out. According to the trace, the applet is actually got launched. This blocked dialog is extra and it is not blocking the applet.
The same applet deployed on no-local server doesn't have such an issue. Looks like only the applet deployed on local tomcat can reproduce it.

Steps to reproduce:
1) Download http://sqeweb.us.oracle.com/deployment2/sheldon/webCases/JavaPolicyFile/JavaPolicyFile.zip
2) Unzip it and deploy it to local tomcat
3) Import cert into User Level Signer CA.
4) Launch the applet from browser.http://127.0.0.1:8080/JavaPolicyFile/html/testSysPropAccessNegative.html
5) Accept the sanbox signed security dialog.
6) If another blocked dialog comes out, bug is reproducible.

Note:
1) if launch the applet from
 http://sqeweb.us.oracle.com/deployment2/sheldon/webCases/JavaPolicyFile/html/testSysPropAccessNegative.html, no such a dialog.