The Bad Gateway (error 400) type of issue is usually a browser communication issue, as opposed to an application issue. Tech sites recommend clearing cache and restarting - but you have already tried that. I had that happen a few times several years ago on a different product and was able to resolve it by using a different browser for a few days. Later went back to Chrome and it has worked fine ever since. I’d give it a try on a different browser if you have one available.
For no apparent reason, one day last week I couldn’t to login to the Admin Panel of my wireless router/gateway from the latest version of Chrome. It reported “This site can’t be reached” “Err-Connection Refused”
I thought I had a hardware problem and I spent quite a bit of time with the supplier trying to diagnose the problem to no avail.
For some reason, I decided to try logging in using Firefox. It worked immediately. Then, I tried Safari. Same thing. It worked. Clearing the cache in Chrome didn’t help. Perhaps the next release of Chrome will fix the problem.