I have been using GIMP for a few days now, but suddenly my brushes have white spots in them. Its hard to explain, so I made a screenshot.
Click image for larger version. 

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On the left you see how it was, the last few days and on the right how it is since yesterday.

I already deleted and reinstalled the brushes, but it didn´t help. I hope someone here can tell me how I can fix this.