Ubuntu – How to fix the connection to api.snapcraft.io

While trying to upgrade Ubuntu from 16.04 to 18.04 I get the following output:

Connection to Snap Store failed

Your system does not have a connection to the Snap Store. For the
best upgrade experience make sure that your system can connect to
api.snapcraft.io.
Do you still want to continue with the upgrade?

Continue [yN]

I would like to fix this before updating. I can find similar problems described online, but they all seem to be caused by snapcraft.io being temporarily down which is not the case right now according to status.snapcraft.io.

(edit: One of these similar problems is Problem with snap store which is mentioned in the comments below as a possible duplicate, but as that question contains very little info it is difficult to know whether the problem is the same and no answers are given that are helpful in my case.)

If I try to run

snap install hello

I get

net/http: request canceled while waiting for connection (Client.Timeout exceeded while awaiting headers)

error: unable to contact snap store

There is nothing wrong with my internet connection. 'snap version' gives me

snap 2.36.3

snapd 2.36.3

series 16

ubuntu 16.04

kernel 4.15.0-43-generic

Any ideas on what might be the problem and how to fix it?