After deleting photos from my phone via computer, there are still empty placeholders in my gallery yesterday I deleted some of my photos from my HTC 10 with the help of my computer.

Later, as I browsed through my gallery, there were spots, where previously been the now deleted pictures. These spots are simply black and if I click on them, there is a loading animation, but nothing happens, and I can't delete or do any other thing with it.

edit:

I did clear the cache and reboot my phone...

and i browse the web, but didn't find anything for my problem
You can try rebuild the media library. Go to Apps in Settings, make it show system apps and locate the app Media Storage, force stop it and clear its data (not cache), then reboot and wait for 15~20 minutes for a full rescan/rebuild.