I'm using Magento 2.3.0, with php 7.2 version, on a ubuntu os. Everything looks working fine, but when I try to go into Magento backend, Content - Design/Configuration - Header, and I try to upload the logo image, I get this:
A technical problem with the server created an error. Try again to continue what you were doing. If the problem persists, try again later.
I have tried already to change the user password, to check the index.php for any echo code, tried also to upgrade, compile, deploy, reindex, clean cache, but nothing, I still get this error..