Your connection to 188.8.131.52 behaves very strangely...
Firstly, the remote web server does not appear to be
using PMTU discovery for you. (I also see this.)
Secondly, because of PMTU discovery not being active,
there is fragmentation *BUT* fragmentation for pppoe is
just bizarre: the first packet is 44 (20+24) bytes long,
the second 1448 (20+8+1420), splitting the TCP header
(32 bytes long) across the two packets. It feels like
someone is trying to put all of the TCP header in one
fragment (but failing) so that all of the TCP data goes
in another fragment, however that is pure speculation.
The remote is also offering an MSS much lower than yours,
1420 vs 1452, but with NATs sometimes messing about with
this field, it is hard to know which one(s) are the real
MSS's being advertised. The smaller advertisement feels
like artificial adjustments to take into account something
else being broken (and the fragmentation above definitely
suggests something bizarre is going on.)
Are you logging all packets that get blocked by ipf?