Yes, it's 5.1.4 where I also have come across the error. I would suggest you upgrade to the latest version of DNN and it should then work ok.
How to backup a live hosted DotNetNuke website and restore it to a localhost installationHow to Upgrade DotNetNuke (Auto Merge Method) (v4.6.2 to v5+)Make sure you create a full backup of your website and database before attempting to upgrade, I would also suggest you test the upgrade on a local copy of your live webiste. The 2 tutorials above should walk you through everything you will need to know.
Thanks,