I found a solution to my problem in http://forum.mikrotik.com/t/problem-to-reach-some-websites/131731/1 Was a really strange thing to debug, cause some sites worked and some others didn’t.
/ip firewall mangle
add action=change-mss chain=forward new-mss=clamp-to-pmtu passthrough=yes protocol=tcp tcp-flags=syn
I have no idea why the MSS only caused an issue with some sites, so I want to learn more about it, but at least the setup is functional now.