Hi Thomas,
Did you upgrade to version 2.3.1.9?
If you visit Admin > System Information does it say you are running version 2.3.1.9?
Did you upload all the new files including the /ClientScript folder?
Did you use the new Web.config file? Do no try to keep an old one, backup your old one and transfer and custom settings to the new one.
Did you visit the /Setup/Default.aspx page to run the upgrade after uploading the new files?
Hope it helps,
Joe