Barney
4
@alzhao and @mainer
Thanks for reading and replying.
I checked it again with a pc directly connected to the internet (without any firewall or router): everything appeared as usual.
Going back to the original pc I disabled adblock on the router (MT300A): everything appeared as usual.
The culprit was the adguard list in adblock, which contains an entry blocking every URL ending in amazonaws.com. Because http://download.gl-inet.com leads to http://s3-website.us-east-2.amazonaws.com the gl-inet firmware download server was blocked, too. Disabling the adguard list in adblock solved my problem.
Barney