On your LMS server close all browser sessions, uninstall all java plug-in (Add/Remove Software), disable any anti-virus software and install the plug-in with the highest revision number you find in the following directory:
NMSROOT\htdocs\plugin;
if you want to test from a client PC, copy the above plugin to the client, and do the same (uninstall all java plugin ....)
make sure, the client can connect to the LMS server on TCP port 42342 and TCP 43242 (e.g. with "telnet 42342")