Access the configuration manager (/system/console/configMgr) using IE11 or Microsoft Edge.
Find a configuration that allows the user to input text inside a textfield. The content of these fields is not visible or editable because the height of the field is set to 0px.
Investigation seems to point to a malfunction of the autosize JS library, as this line in config.js set the height to 0:
A possible solution would be to set a min-height on the textfields, if the issue in the library isn't fixed.