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https://fogbugz.unity3d.com/default.asp?906743_2go0o2dfsri25vhb They confirmed that this is a bug.
I think this is bug of Unity 2017.1.0b2. In the following more stable versions it should be fixed. Now the best solution is to use Unity 5.6.
I have the same issue in Unity 2017.1.0b2. Changing canvas Render Mode to "Screen Space - Overlay" solves this assertion.