Over the last couple of days, three different people asked me a similar question related to forms stuck while loading.
Even though they formulated it differently (or even forgot to mention the environment got upgraded). It always came down to:

After an upgrade from Dynamics CRM 2016 to Dynamics 365 my form is stuck on “Requesting data from Dynamics 365.”

Requesting data from Dynamics 365


So in these cases the solution was always, clearing the browser cache and reloading the form.
Since a refresh using F5 should be enough, after all we already cleared the browser cache.
Most of the time will do a  hard refresh using CTRL+F5. Since this will bypass the cache, better safe then sorry.

More information on clearing and bypassing the browser cache is available at:

If the problem persists use private mode or incognito (for the chrome users) to rule out any caching problem.
And remember F12 developer tools is your friend!