Zweitze,
thanks for your thoughts! Unfortunately this is an account that we manage for a client, so I do not have access to the webserver logs or firewall configuration. The URLs are plain HTTP, so that's probably also not the culprit.
Something I forgot to mention is that when I select a random one of these disapproved ads in the web interface and use the "Test" button, it consistently shows that URL does, indeed, work. As far as I know, this also checks for final URL mismatches like cross-domain redirects and the like.
Of course, the fact remains that the Adwords crawler for one reason or another could not access the landing page, but I find it curious that we're experiencing this in a much greater frequency than, say, a few months ago.
Thanks again and best regards,
Dorian