This are excerpts form
http://renaud.waldura.com/doc/freebsd/pppoe/ - section 6.3:
A potential problem with the PPPoE configuration manifests itself in the following manner: client systems become unable to access certain Web sites. A Web browser stops in mid-download of pages, or does not access the site at all. Initially quite puzzling, it quickly becomes a major annoyance.
I ran into the problem with various sites, netscape.com, wellsfargo.com and paypal.com. The sites are accessible from the gateway machine, but not from client systems located behind the gateway. A network trace with tcpdump does not show anything abnormal, apart from no packets making their way back to your system.
But let's explain what's going on. Your client systems implement Path MTU Discovery by sending TCP packets with the "Do not fragment" bit set and a large TCP Maximum Segment Size derived from the local MTU value (a complete description of Path MTU Discovery can be found in RFC 1191). Since your clients are connected to an Ethernet, their packets are sent with a MSS = MTU(Ethernet) - 40 = 1460.
Unfortunately, because of the additional encapsulation, your PPPoE link has an MTU smaller than the Ethernet MTU. It is not an issue on the way up, from your client to the Web site, because the client is only sending a HTTP request, which typically fits inside one packet, smaller than the MTU. But on the way back down, the web site is sending large packets (the Web page, or the file, that was requested).
Whenever a data packet larger than the MTU (for the PPPoE link) is received on the ISP router, it drops the packet (the router would fragment the packet, but it cannot because the "Do not fragment" bit is set) and sends a ICMP "Fragmentation needed" message back to the originator (the Web site), telling it to resend smaller packets.
Unfortunately, if that ICMP message does not make it back to the originator of the packet, or is not generated by the router, the originating Web site will not resend those packets, and they will never be received by the client. Sometimes the problem is blocked ICMP traffic, sometimes it is a misconfigured ISP router that does not send ICMP "Frag needed" messages. The end result is the same: the client does not receive the Web page it requested.
So, this is not a KDE or Konqueror issue!
Best Regards,
CASIUS