Does it always happen to the same images, or is the problem random and occasional...?
Perhaps post some files: source (with xmp if applicable) and exported file. (maybe Screenshot too if you know how).
Is the exported image displayable using Windows 'Preview' or any other viewer? Can you load the file into Photoshop, or any other editor? If you know how, it would be interesting to see what exiftool says about it too (source, xmp, and/or exported...), or ImageVerifier...
I take it Lightroom did not issue any warnings during export?