URL with colon flagged as 404 in SEMRush

SEMRush reported that our website has an odd 404 error tied to a page that is our website + a colon.

For example, it flagged this url: https://[insert_domain_here.com]/:

But there is no such page as that. And never was.

Why is it flagging our website followed by /: and how do we resolve?

Please advise. Thanks in advance!

Here is my site Read-Only: LINK
(how to share your site Read-Only link)

Your website link and readonly?

@memetican the url is guninsurance dot com.