Here are some pictures so that it's easier to understand what's going on.
The image upload in front end that is stuck on 100%:
[https://ibb.co/SfB2sCf]
Uploading the same picture in media folders in back end (the upload was completed and did not get stuck):
[https://ibb.co/Xxpvz12]
The image "stock-img", which was the one I downloaded, is visible in the images-files in back end:
[https://ibb.co/mFzM2Y3]
The image "stock-img" is nowhere to be found in the images-files from front-end:
[https://ibb.co/55k0P1H]