After upgrade to 4.6, Preview in admin not working, but works outside


Hi All,
I just upgraded from 2.1.1 to 3.1.1 and 4.6…
Most of the stuff upgraded smoothly.
My biggest issue is in previewing objects.
While I am in the admin panel, both extJs 6 and 3, I see all the info there.
Once I click “preview” I get : "The requested URL /my/object/url was not found on this server."
When I open the iframe in a separate tab:
it works as expected and I see everything without errors.
I cleared all caches on pimcore, restarted browser, cleared browser cache, but still the same
Any advice?


Are there any more details I can add to be clearer?