has anything changed on the server since uploading worked with imagemagick? has the server been updated?
As uniform feature notes it is only a imagemagick thing you can not do it with GD2.
If you can not get a functioning imagemagick install then you must be GD2 and use normal thumbnails.
Our software has no control over the server image processors so you need to use which ever one works.
You can try to recompile Imagemagick on the server if you have root access otherwise you need to ask your host to make imagemagick work.