internal server error when deleting photos

Hi everyone, I ran out of storage space on my icloud account and now want to clear up space.

First problem: apparently to much data... As I don't want to download every pic separately, I installed the icloud app on my PC (Win 10) a day ago - but nothing happened, only got the message "photo storage is prepared, that may take a while with lots of data" (had to translate, as I get those messages in German).

Then I thought - oh, wait, they tell about "new pics" for the download. So I took a new pic, and suddenly there was the option to decide if I wanted a yearwise download or everything. Opted for everything, got 5 (shared by others) pics out of 800... Probably not patient enought, but no chance to see if something is working in the background.

So back do downloading one by one via the icloud website - but a new problem arose: Not all of my pics are deleted when I try to, some encounter an "internal server error". So far only those that I uploaded two years ago manually.

How do I delete those pics - so far my solution would be get everything out of icloud photo storage, than kill the app on every device? Or is some nicer option available (perhaps even a nice option for the download)?