Error when I upload photos to the Gallery

Hello, since a couple of weeks, we have issues when we went upload photos to the gallery on a private website. I already looked in different forums but no solution fixed the problem.

When I want to upload a picture, I get wrong message: The response is not a valid JSON Response.

Maybe anyone have an idea what I can try to fix it?
I checked already the permalinks, the url and the plug-ins.

 

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