esmi, thank you for your answer!
We asked our host provider, they said, that the problem is not on their side.
Maybe I should explain more about how I got the problem (checked 3 times, always the same result):
– we have a site.com, than we copy it to the subdomain.site.com (files + database upload)
– everything works correctly, image uploading too
– we delete sll the images in the media library (we tried 3 different ways: with FTP, in the Administrator panel Media Library or with different plugins)
– after that we got an error with uploading images
