So I had a case where I needed to get the value that was in the people picker field on page load, store it, then compare it if/when it was changed on the page prior to saving - and if so, set a checkbox field on the page so a workflow could grab the new person field value and send them a notification. I just used a REST call to get the value, initially, ...
Check the Port number of the web application and try to perform iisreset.
A similar post:
SP 2016 on -premises error: Sorry, we're having trouble reaching the server
Another blog for reference:
We’re having trouble reaching the server SharePoint error [FULL FIX]