I was having trouble viewing Java applets in Ubuntu (hardy) earlier:

java_vm: xcb_xlib.c:82: xcb_xlib_unlock: Assertion `c->xlib.lock’ failed.
Could not read ack from child process
Plugin: Java VM process has died.
plugin: java process died due to signal 6

The solution is to set the following environment variable in the shell beforehand.

export LIBXCB_ALLOW_SLOPPY_LOCK=1

This applies to appletviewer too.

Update: this is another possible solution (I haven't tested it).