Downscaling Image destroys it

Hi everyone!

I have a high resolution photograph in media. Using the full resolution in my post is too slow, but the automatically generated medium size is too small, thus I used the media > scale functionality to reduce the image’s size to a set value. I get no error message, but once it’s done the image is replaced by the small ‘image couldn’t be loaded’ icon. I can roll back the changes, but resizing just doesn’t work.

I am using the autofocus theme, and only have Akismet, Cloudflare and Jetpack as plugins installed at the moment.

UPDATE: After trying several times I found out that sometimes the problem is as described above, and rarely the scaling just loads indefinitly.

Also, the photograph’s resolution is 3314×4978, I’m trying to go to 800×1202.