For a short time Apple made the 7.1 beta version of Safari accidentily available also for non developers.
You will find more details regarding this here.
This version of Safari can cause that Documents in Mail Designer 2 cannot be opened.
You will see the version of Safari you have installed on your Mac by choosing "Safari > About Safari" from the top menu bar.
If you are using Safari 7.1 on your Mac, please install Apple's 10.9.4 Combo Update that can be downloaded from the Apple website and will install Safari 7.0.5:
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