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I would estimate that Google's a bit overloaded, following an unexpectedly high response to yesterday's bulletin.
Getting PSA's here also, but only on homepage, internal pages all seem to be fine.
This has been happening to me since late last night. All other pages of my site show ads fine, but nothing but PSAs on the homepage now. Plenty of ads available to be served, I'm sure, as all the other pages of the site are the same topic as the homepage. Very frustrating. I changed the ad slightly this morning hope to get mediabot to wake up. No change yet.