Strangely when I installed the Advanced Custom Fields plugin also by the Elliot Condon it corrected the problem. This is not really a fix. What I've deduced, although I can't find an official announcement, is that Custom Field Matrix is no longer being supported by Elliot. I recommend ceasing to use this plugin and migrating over to Advanced Custom Fields. It's got a great eco-system which includes very useful paid add-ons. It's too bad because there's some things I really like about Custom Field Matrix. Elliot appears to have taken what was good about CFM and migrated these to premium features in ACF (things like the Repeater).
In short, unless you're a hardcore developer and are going to reinvigorate this plugin and keep it alive I recommend staying away. It will break your heart as it's quite good...but it hasn't been maintained in a year and it's starting to show.