Screenshot Monitor uses standard system API to take screenshots. 3rd party software/drivers that alter this API or add malfunctioning hooks to system can be the reason you only see black screenshots.

Check if you can get correct non-black screenshots taken with other software? For Windows 10, it can be as simple as pressing PrtScn, for other - you can press PrtScn and then paste the result into Paint program or any other image editor.

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