Thanks for your reply giliard!
I have Ghostery installed, but always set to "paused". This problem does also happen for my colleagues with different Setups (most of them have no AdBlockers installed), and only in Chrome.
At the moment this occurs far less frequently then yesterday, so maybe, whatever caused it, is resolved.
We will also check if there is maybe some software on our PCs that may have impact on the displayed ads.
But from your side, there are no known issues that could be causing this to happen?