while being on travel for work, suddenly I was unable to reach my home-site anymore. When I came back, everything was still running, except the internet connectivity would not work, and the reason was unknown.
I first suspected an issue with my ISP, however testing it with them, it was not the ISP's fault, rather on my router. However - identifying the issue avered a pain, as it was not the a fatal problem, rather a non fatal.
For the router, the interface was there, and it tried connecting to the ISP (ADSL), however never managed to get an answer. Seems something was not working right. At first - I though the flash-drive failed, so I reinstalled the router, and restored an old backup (2 Weeks old), Didn't work either. I then had the idea to disable the PPPoE interface, and use another interface of the 9Port Router (RB493AH). And this time - it worked.
Needless to say, this entire operation has taken me most part of last night ...

So to make it short, on port on my router is broken, the others still work. I'll make sure to have a backup router at hand for the next time this kind of issue happens ...