First, I would recommend disabling all third party applications/plugins to see if there is a conflict. From the error stack, I don't believe so but a good first step to confirm this is indeed coming from an issue rooted in our core software.
Second, please check to ensure that the uploads folder and all sub-folders of uploads are writable on your server. This is generally 777 but if you run suPHP or suexec this is 755. Please also ensure that the correct user/group is assigned to all folders/files in your installation directory. If you are unsure how to perform any of this, please reach out to your hosting provider. Once this is done and all are set correctly, please clear cache by going to ACP -> Support (top right) and then on the right-hand side click "Clear System Caches."
Please let us know if that does not help.