Problem: disabled widgets in older Glade files

It may occur that certain widgets in Glade are greyed out after opened in Glade.

A hint is hown in tooltips like

The cause for this behaviour is the Gtk+ version given at the beginning of the file:

<?xml version="1.0" encoding="UTF-8"?>
<!-- Generated with glade 3.20.1 -->
<interface>
  <requires lib="gtk+" version="3.0"/>
  ...

This line will not be altered even if the file is saved by a more recent Glade version. So you just manually replace "3.0" by a current version ("3.20" at present).