Hi, @ftiede,
I did some troubleshooting from my vantage point at NTS Workspace AG (AS15576) in Switzerland, and everything looks okay from there. DNS for ocsp.int-x3.letsencrypt.org gave me a different Akamai POP, but I was able to connect both to it and to the same POP you were given (2a02:26f0:e7::5f65:48c0, Telecity Frankfurt).
In our experience, Akamai won’t usually be able to work directly with us on routing issues with individual ISPs/ASNs like this. They’ll want to talk directly to the affected ISP’s network engineers. I’m sorry about the trouble, and thanks for checking with us!