Did you install the plugin? I think you are still on a Discourse version before it was included in core.
This is a version from July 16th.
And the ad plugin is included since the 22nd.
While there are new commits almost daily, Discourse only notifies you about updating when there is a beta version bump. It’s easy to forget that.
You nailed it. Thanks por you help, solved! 
Has anyone been having problems with banners being larger than intended and overlapping content, specifically on mobile? We use DFP and this has been happening more frequently lately.
On DFP, we have created multiple ad units with a specific fixed size (not fluid), one for each position/platform that the plugin supports, in order to be able to specify just that exact size, and yet, it doesn’t seem to be enough to keep it from rendering in unintended sizes.
We have disabled “optimize ads” on AdSense (which DFP uses as backfill) and, as far as I can see, DFP does not have any setting to configure automatic ad expansion that I can access.
For whatever reason, it seems to happen mostly (if not only) with the ad units that are placed lower in the page (e.g. below the fold) — it’s specially common with the “above suggested” placement, but sometimes it also happens with other placements if they happen to not be in the viewport on load. Hence, I’m guessing this is happening when they show using .refresh().
The plugin seems to use SafeFrame which I think should make it render on an iframe of a specific size so this shouldn’t be happening. And yet… this is happening fairly frequently. We use this same DFP account on a larger website and the ads never render bigger than intended. Unsure if relevant, but on this other website, we do not use explicitly use SafeFrame.
This is tricky to debug as it does not always happen (normally the right size is used), but on the aggregate, it annoys quite a bit of people. Has anyone experienced this? Any idea what may be causing it?