Hi @Rikard ~ I think it’s definitely worth it for them to try that fix, I’ve now seen it work on three different sites!
We are experiencing similar issues re: uploading images to the Media Library. We did find that we were able to upload an image if we first removed all non-alphanumeric characters from the image file name (for example, changed “Ultimate-5-Plus-5.jpg” to “Ultimate5Plus5.jpg”). Prior to WordPress 4.5 we didn’t experience this problem. We will keep an eye on the thread for a resolution, thanks!
*****UPDATE: The fix for us was found in the following thread: https://kriesi.at/support/topic/upload-image-giving-http-error-theme-error/
See Josue code fix, reply #514811
Thank you Kriesi for the great support forum!