load of errors in my Robots.txt
If those are the only lines in your robots.txt, why not try emptying the file entirely (or deleting it) and seeing if that resolves the errors.
You're basically saying "don't disallow anything", which is the default when robots.txt is empty or missing, anyway.
If you are intending the * to be a wildcard, that format does not look correct to me according to the robots.txt standard. However, Google does pay attention to some nonstandard robots.txt enhancements, which I'm not familiar with.
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I did some more hunting. I do not use the "authorized sites" feature, so I don't get warnings. However, in AdSense control panel, I do see numerous "crawl errors" listed, all of them blamed on Robots.txt.
However, the URLs (that the AdSense crawler supposedly tried to crawl but couldn't) all start with these (and possibly a few others):
hxxp://webcache.googleusercontent.com/search?q=cache:...
hxxp://www.bing.com/images/search?q=...
I don't block any of those in my robots.txt -- in fact I can't because those URLs are not on my site!
So it can't be MY robots.txt that the crawl errors are complaining about! It would be the robots.txt files at the referenced external websites.
Maybe that's a partial lead toward an explanation?