Rebuild failing " too many connection resets"

while making an an upgrade (Ubuntu 16.04.06, older installation, not updated for at least 2-3 months)
on CLI via “rebuild app”, i was stuck since several hours,

su discourse -c 'bundle install --deployment --verbose --without test --without development --retry 3 --jobs 4' 

was rendering a message, that the ssl was failing while fetching files on

(nevertheless, it was not always the same file which failed, so i assume there was progress during some attempts)

After several reboots of the VM, disabling IPv6 etc, i reduced the MTU for the system to 1280 (just a shot in the dark): Voila!
So some kind of PMTU-detection failed on me.

(in have no explanation, this is just a mention, perhaps for others who are stuck on a similar issue.) periodically rate limits different hosts/subnets. It’s not something you did, and it’s unlikely the MTU change fixed it. There are lots of people running into this unfortunately. Glad it’s fixed for you now though.