The ads suddenly stopped showing. I checked the site with firebug and it shows that the ad group is being rendered, but for the images I get the following:

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iam_serg’s picture

iam_serg CreditAttribution: iam_serg commented
Issue summary: View changes
Michael_Lessard_micles.biz’s picture

Michael_Lessard_micles.biz CreditAttribution: Michael_Lessard_micles.biz commented

In my case (Feb. 2015), it was the file caching option that made my "Ad" banners vanish. I turned off the file caching "Ad" option, and they reappeared.

This info might help others ...

ps: I still need to find out the actual reason/cause and fix it.

iam_serg’s picture

iam_serg CreditAttribution: iam_serg commented

My case was that the error log on the ad module folder was 1 gig. Once the module can't write errors to error log, the ads would disappear. Hope this helps.