Thumbnail and other size generation off

Recently we have moved our website to a new (virtual) server. Everything was running fine after some fine tuning, except one problem that keeps us busy thinking about a solution.

When uploading a new image, we seem to be unable to insert the image into a post with just the original size! No thumbnail generation is possible and also the other ‘middle’ size isn’t possible. We can’t tick the box for other sizes. Maybe the picture below gives you a better idea.

Anyone know what to do in this case?

View post on imgur.com

Any help is very much appreciated!

Eric

[ Please do not bump, it’s not permitted here. ]