Mark,
You're not that late. Security updates still come out for Joomla 1.5, we still support it (for a few more months at least... announcement coming soon) and, honestly, this site is still 1.5

So you're not late, your wise!
As for your issue.. yup, definitely something wrong there. I'm getting a 500 server error when I try to login. Can you try the following:
* Turn of RokZipper (if enabled). Go to Global Config and set Error Reporting to "Maximum" and turn GZip compression off. Then, try to login and see if you get any better error messages that may help.
* Disable any JFBConnect profile plugins you have enabled and try.
If that doesn't help, let us know, and we can come up with more. Not sure what it could be, but we'll help you get to the bottom of it!
Thanks,
Alex