Can’t upload a featured image

Hi! I can’t upload a featured image in any article in my blog.

The error says: The server cannot process the image. This can happen if the server is busy or does not have enough resources to complete the task. Uploading a smaller image may help. Suggested maximum size is 2560 pixels.

I tried different photos with different sizes etc. but nothing happens. What is more, older photos that are saved in our library can be uploaded without a problem. But when I try to add the same photo, but upload as a “new” it shows that error.