I just wanted to confirm what Steve has said. The actual reason was that we have found some breaking changes. We are working on resolving those. Unfortunately, at this point I cannot say with certainty will the update happen with 5.1 or with a possible Service Pack 2. However, I want to assure you that we do understand that this is a problem and are working on resolving it.
Also, starting with Sitefinity 5.1 Sitefinity and Telerik Open Access will be released in parallel. We have setup a joint testing environment where the latest drops of Sitefinity and latest drops of Telerik Open Access are tested daily with test suites of both products, which will give us early feedback and allow us to release in parallel.
the Telerik team
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