501 error has started appearing frequently

I administer a web site that is maintained using Dreamweaver CC (2017). Very recently, I've been getting several reports of people getting 501 errors when they try to visit my site, which hasn't been the case previously. Each time, I believe they have been using Chrome as their browser. Changing to another browser has taken care of the error. Any ideas on what types of things might be triggering the error that are browser-specific like that?