“Image.jpg” has failed to upload due to an error Failed to write file to disk.

I’ve an issue uploading images to the server. It throws below error message :“xxxxx.jpg” has failed to upload due to an error
Failed to write file to disk.
I’ve checked that the directory has write access. I’ve two seperate installations on the same server. One of them is multisite network and the other one is stand alone installation. Both of them have issues uploading files to the server. It was working fine until last week and I believe nothing has changed. Any idea how can I fix the problem?. I would really appreciate any help.