I successfully uploaded the Image (Sonnenuntergang.jpg).

I am quite shure that it is not to do whith the image because we tried uploading images that we allready uploaded before our update.

It can't be the Max Image dimesion either because I croped the Image in Photoshop before and checked that it is no weird Image format. Its a standard JPG.

It can't be the format because we made shure that in the list of allowed formats JPG is included as well as GIF and PNG etc...

I don't know where else to look. Its probably just some tiny setting hidden somewhere ...