Oct 09, 2018 06:16 PM|mgebhard|LINK
It is hard to provided assistance without the source code or code that reproduces the issue. The error message starts at <!DOCTYPE html P'. which indicates there is a problem with the HTML. The tag should look like this...
Use the browser's network (F12) trace to see what's being sent/received. Also check the browser console for script errors.
IF you are using telerik controls, see the following.
or Google in general