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I'd like to add to this. I'm experiencing the same issue of project corruption, but in my case it's using special characters in any text field (for example in a quiz question). When i copy over some mathematic content from word some characters aren't properly interpreted in Xerte and a unicode unknown character (white square) shows up. When i save and close the project without removing this unknown character, the project becomes corrupt and will show blank screens like OP's screenshots.