I suspected that something was wrong with my configuration, since I did not have to suddenly update ADT when everything was before.
By the way, I got this with the "Android ADT version required 21.0.0 and higher."
To fix this, I:
- Reinstalled eclipse (which is pretty simple since zip was still in my downloads, so I just deleted the eclipse directory and unzipped the original again - eclipse does not use the registry)
 - Created the java application "hello_world" in eclipse to check this (fixing any problems with standard searches ");
 - An Android application "hello_world" was created to check this (fixing any problems with standard search queries).
 - Returned my project files to the latest known working version,
 - Imported my project, fixed problems, etc.
 
About an hour of work. No downloads (assuming you still have your eclipse zcl), no updates to ADT, and no obscure issues that I have not encountered before.
This person (http://kcwebprogrammers.blogspot.com.au/2012/07/android-sdk-eclipse-hell.html) recommends a full reinstall, but it seems that a partial recovery may work.
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