Could you try enabling developer tools and check whether there are any script errors present in console. Also as you have upgraded from 6.X, make sure your browser is not running in compatibility mode as Pega 7 doesn't supports compatibility mode.
We already check all things above mentioned , we created SR also for this. They are also not able to find root cause of this, So they suggested to bring this in forum. May be someone already faced this kind of issue.
In your userworkform you have a method called AjaxRequestFail. It is attempting to output the status of an unexpected error. but it looks as if someone mis-coded that method. Confirm if you have this method, and see if it is obvious how it should be fixed.
As to why you are getting this error, it depends upon what the low-level network error is that you are seeing. If it is one for which you don't care, you can modify the method to skip the output when the error code is what it is..which, currently, we don't know what it is.