HTTP error 500 indicates a server problem. You should report the problem to the owner of the website so they can fix it. Some sites deliberately behave differently for different browsers and OSes, so that could be triggering the problem at the server.
There's one more possibility, if you're using mobile data (instead of Wi-Fi) to access the site. Most carriers use a transparent web proxy to compress data you access on the web (especially pictures), to use less bandwidth. Obviously this only works when you're using HTTP: for HTTPS, the carrier's proxy server doesn't know what the page contains. If you're seeing the same problem on several sites, it could be that the error is coming from the proxy server (or is otherwise caused by the proxy server), not the site you're trying to access. In this case, another phone on a different network wouldn't show the same problem. If you're using mobile data, and you suspect this problem, you can check whether it happens on Wi-Fi too.