Update: no news from HostICan, but if anyone is having problems
and is feeling
and can figure out how to do it, they can try the following, from
http://community.acanac.com/acanac/viewtopic.php?t=3508:
The solution to this problem was to set the MTU to below 1450, and the MRU to below 1450.
It would appear that HostICan aren't the only provider having problems with networks peered via Cogent. Hopefully this isn't a resurgence of the peering dispute between them and Telia from a few years ago which was very disruptive.