Method 'get_Cookies' in type 'Servicestack.host.aspnet.aspnetrequest'
We just started seeing the following error when editing some content items. We are on Sitefinity 7.2.5324.0
The error you are getting is related to your .Net version. You can see some additional information on it here.
In order to get rid of the behavior you will need to point your Sitefinity instance to the .Net 4.5 framework. To do that, please open the SitefinityWebApp project in Visual Studio, right-click the project, select Properties. On the Application node you will see the Target Framework drop down. Select .Net Framework 4.5 and save you changes. Restart the application. This should eliminate the behavior you are experiencing.
I hope the information above was useful.