Error 500 is internal server error, so unless either of the below fix your problem, you'll have to speak to someone in support.
My first thought is, do you hide behind a VPN? If so, try turning that off and see whether you can login. I use Express VPN and if I keep myself in UK - Docklands for example, I get error 500. I'm not sure whether Upwork are trying to place an outright ban on VPN's or whether they just banned someone who was using that IP address.
If you're not using a VPN, I would recommend clearing your browser cache, as there may have been a blip on the Upwork server when you initially tried loading the page and that may the 500 error page may have been cached.