The problem is caused because since version 5.1
A/B Campaigns are no longer accessible
via Email Campaigns -> A/B Tests page. This page should be hidden
from the menu after upgrade and even if you try to access it you should receive error 404. You can see how to access the A/B Tests from this help article
To fix the problem perform these steps.
1. Navigate to Administration -> Modules.
2. Locate the Email Campaigns entry.
3. Deactivate the module.
4. Once deactivated, uninstall it.
5. Install it again.
After that you will be able to use the Email Campaigns module successfully. The content is also saved, so you will not lose any email campaign.
the Telerik team
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