Hello,
Please inform what menu item are you trying to upload image?
I have tried to upload an image but your Media Library is empty shows images and then they all disappear.
Please deactivate all the third parties plugins not related to the theme
In order to distinguish which plugins relate to the theme and which do not, deactivate them all and then navigate to Appearance > Install plugins and activate all in the list.
Next, activate one by one and check the issue each time to find out what plugins cause the reason.
Best Regards