Would it be possible to check the System.config file to see if the Forums module was properly upgraded and is on version 6 as well?
You can try the following on a backup copy of the website: From the modules menu, disable the Forums module, after that uninstall it from the same. Once it is uninstalled, restart the application, install it, and restart it again. If the issue persist try the same for the Feeds and Notifications module.
If the problem persist, we will need to take a look at the project locally.
All the best,
the Telerik team
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