Very old client we did a pro-bono site for tracked me down and is having problems logging into the control panel of their EE 1.6.8 site. When the login is filled out and submitted it just takes you back to the login screen. I've seen this before and usually just clearing the cache works, but it's not working in multiple browsers even after clearing the cache. Any ideas?
Go to your
config.php file. If you're using cookies, sessions, or both, switch to a different option. For example, if you were using sessions, switch it to cookies only, then try to login.
I'd also check with the ISP, to see if they updated the servers version of PHP. If the client isn't prepared to upgrade to EE2, then at least they should upgrade to EE 1.7.3. I know between 1.6.8 and 1.7.3 compatibility for MySQL 5.5 was introduced, so it's also possible that some PHP compatibility issues were also addressed.
The thing is, with software like pMachine or EE 1.x, while they are pretty solid pieces of software, that don't need to be upgraded (generally speaking), they also run on servers, that will be continually upgraded (if the ISP is responsible), which means they will eventually stop running. So upgrading to the most current version you can is good.
It doesn't have to be EE 2.6.1, but EE 1.6.8 should probably become EE 1.7.3, with a warning that they'll probably need to migrate to EE2.x sometime within the next year or two.