It might be a problem with your WP Theme but on a side note, I wouldn't worry too much about Microsoft Edge. Less that 6% of the human population use it simply because it is one of the most dangerous programs ever created. Microsoft knows this and since there are millions of Windows users who are clueless, they figured... hey let's just change the name of Internet Explorer (IE) to Microsoft Edge and hope that no one notices.
Go to this link and scroll down to December of 2002. At the time about 84% of all humans used IE. Then scroll up to Sept. 2016. Look at the stats!
The reality is that 73% of the human population uses Chrome. My suggestion to you is to do the right thing and instead wasting time trying to get your forum to work for IE (Edge) users, instead help them out and redirect them to a page explaining what I'm explaining to you right now. Do them a favor and get them to use Chrome as their default browser. Better yet, inform them at the definition of true love is a Mac and sway them to never again buy another Windows PC.
You want your users to be savvy and less prone to viruses and malware so maybe this a great opportunity to install a script that sends everyone who is using IE or Edge to a page informing them they're playing with fire and get them to install Chrome. Don't forget that almost every virus or malware ever created, was designed to infect a PC using IE. That's why it's considered one the most dangerous programs on Earth and why no one uses it any more.
Hopefully Robert of another mod can help you get to the root of the search problem you're having but at the end of the day - consider what I'm telling you. Help your customers (visitors) and pay if forward.
Thank you for letting us know, we'll work around this issue.