Looks to be a java plugin issue that I've seen quite a few times. What version of Java is running on these problem clients?
PDM Version 3.0 supports the
Java Plug-In 1.3.1, 1.4.0 and 1.4.1 (recommended) so make sure the clients have a supported Java plugin version.
You could also upgrade to PDM 3.0(4) to get newer java support.
Download the file "pdm-304.bin"
Cisco PIX Security Appliance Software-Download
http://www.cisco.com/cgi-bin/tablebuild.pl/pix