Here you can see same question is open joomlart.com/forums/d/34412-screen-shot-error/3
and waiting for your reply.
Kindly edit fields in first post and submit temp super user login i will check if its changed. If you ate getting same error, its means there is no limit change as Joomla still showing you limit is 20MB only.
OR
you can try to install extension using Joomla > extension > install from folder method.
docs.joomlabamboo.com/getting-started/how-to-install-items-using-the-tmp-folder-in-joomla-3.html
