A few of my members are having this problem, yet everyone else is able to log in perfectly fine and use the site as usual. I thought it was because the .index/php wasn't at the end of the URL but even so, they are having issues even with the .index/php and the link should redirect to the index page automatically.
This is the screenshot they sent me.
Any ideas why this could be happening? (I usually don't work with the server, another admin does that, so if this is a server issue, please forgive me for posting here.)
The error that is giving you is a pretty good indicator on what is wrong. The DNS entry that converts your domain name to an IP address is not being found. It could be an issue with whomever you are hosting your DNS with (unlikely if others can get it), or with the ISP (the users internet provider) being used typically. The latter is the most likely issue. It could also be down to the router you are using, especially if it is providing the DNS for your home service.