The problem I mention has to do with MTU size discovery. Sometimes it works, sometimes it doesn’t.
The problem can be inside your network/router or it can be elsewhere along the path.
Some people block all ICMP after they have read some clueless advise from people like Gibson, and they cause such issues.
Other times MikroTik break the automatic MTU clamping in PPPoE and suddenly it does not work while it worked in a previous version.
You can try adding this to the configuration:
/ip firewall mangle
add action=change-mss chain=forward new-mss=clamp-to-pmtu passthrough=yes \
protocol=tcp tcp-flags=syn
When it fixes your problem, it means there is something wrong somewhere else, but at least you have worked around that.
In severe cases you could even try something like new-mss=1400 instead of new-mss-clamp-to-pmtu.