Some http POST requests fail

I have been having a strange issue on my home network where some http POST requests fail to finish. I have noticed this issue on 2 websites so far. When I try to edit a wikipedia page the edit page loads but when I hit send the post request gets no reply, the get requests all return 200. I also have this issue when submitting edits to openstreetmap.

I don't have this issue on other websites like reddit or here.

I have tested this on a few computers on my home network and they all have the same issue, my desktop at work is unaffected however (All computers are running ubuntu).

On my home desktop I submitted a change to opensteetmap through the tor browser and it sent fine but when I try in my regular browser it fails.

I'm lost as to where I should check now as I cant see how some types of requests could work and some fail on a single website with https.

Solution 1:

Your screenshot shows a 20.4KiB POST failing. See if POSTs of less than 1KiB work. If they do, you may have a Path MTU Discovery black hole on your network. Work around it by setting your interface MTU down to something low like 1KiB, then working it up higher and higher until you find the max value that works.

PMTUD black holes are usually caused by firewalls managed by idiots who block all ICMP because they think it's only used by ping. Sometimes they're caused by bad NAT gateways that don't know how to handle ICMP properly. Sometimes they're caused by bad routers that don't know how to handle the IP header's "Don't Fragment" bit properly.