Custom Mixer Brush Tool brushes are not working

[JH1229]

I've been making custom Mixer Brush Tool Brushes without a problem for a while now. All of a sudden, I'll make a set of brushes, test them, and once confirming they work, exporting them. When I go to use the brush, I select it and it shows the brush shape in the preview box on the top tool bar but as soon as I click my mouse to use the brush the preview box seems to reset to nothing and won't work. I can't for the life of me figure out why this is happening. 

[c.pfaffenbichler]

Could you please post screenshots with the pertinent Panels (Toolbar, Layers, Brush Settings, Brushes, Options Bar, …) visible?