Hi Faye,
This seems to be some bad web.config configuration (of .NET itself). Is this happening on an OutSystems Cloud environment (Enterprise or Personal) or is it on your own On Premises servers?
If it's the first case I'd recommend opening a Support Case with OutSystems, if it's the latter, you need to talk with your SysOps and have them check that web.config and eventually validate that it's not something they may have skipped or mistyped while following the OutSystems Installation Checklist for your platform version.
Hope this helps!