dport,
on my xampp system, I actually downloaded the slightly newer version of the PMA package and set it up from scratch, so I never went through the configuration on the bundled version. I have noticed, however, that my config.inc.php file in the newer version that I created using PMA's own setup scripts, is MUCH shorter than lampp's version: (45 lines vs 827 lines). I haven't had any problems accessing it from outside the network via domain name, so perhaps this might be the quickest solution for you to try -- you may have to edit the httpd.conf file to make a convenient alias to the newer pma folder, but you can turn to PMA's strong documentation for help setting it up.
If you'd rather not do that, I'd suggest we first confirm that it's a session-object creation issue:
[note: I'm referring to the /tmp directory below, but if you don't see what I'm talking about there, look in other temp directories, like /var/tmp]
1. Go to your /tmp folder and clear out any "sess_*" files (assuming you're not running any php apps on the server that users are currenty logged in to).
2. Open PMA via
http://172.28.150.250/phpmyadmin.
3. Confirm that a /tmp/sess_* file was created. (If you're curious, you can open it up and see a few session objects in there: PMA_token, PMA_config, PMA_Theme, maybe more, all listed on one really long line -- the reason I think it's a session issue you're having is that the php line reported to be causing the fatal error is choking when attempting to access an object that doesn't exist, which it is ultimately trying to pull from stored session variables that are created when you first visit the PMA site)
4. Now close the browser, delete that sess_ file, open a new browser window, and goto http://{servername}/phpmyadmin. (you still get that same fatal error, right?)
5. See if a new /tmp/sess_* file was created. Please tell us if it was created, and if so, what are the contents? If the contents are really long and you can't post it all here, just list all the obvious "PMA_*" names that appear before each pipe "|" character in the file.