Deleting ".android" is temporarily fixing the problem with me as after sometime it begins to come again ..
Here is another cause and fix that might help .. This is happening because of existence of 2 versions of SDK in the same machine but in case you have only one version (probably it is 21) just download the SDK platform for API 20 (4.4W) as there is a problem in 21's "layoutlib.jar"). start applying the following:
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Close Eclipse
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Navigate to your sdk folder (something like C:adt-bundle-windows-x86_64-20140321sdk)
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Go to platforms folder -> android-21 folder -> data folder and rename layoutlib.jar (for backup only)
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Copy the same file (layoutlib.jar) from your android-20 folder to this folder "android-21"
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Start Eclipse
转自Stack Overflow
这个问题烦扰了我好几天，用这种方法终于解决了。