Thread:Project:Support desk/Error creating thumbnail: Unable to save thumbnail to destination (2)/reply

Well, that may be too obvious, but in case it has been overlooked: do you have enough free space on the server to upload files? Becasue if nothing has been changed and it was working before, that's the first reason it comes to my mind. That, or a disk quota imposed by your host.

If not, set a debug log as explained in Manual:How to debug when performing an upload, and disable the log again after the test, and look at any relevant info when the error happens to see if there's more information about the cause.