Is this true for any version of 5.x? E.g. did you try the 5.4.0 release from download.slicer.org? Extensions from preview versions are not kept forever. Also what OS are you using?
You might also have a network proxy so maybe you can try on a different network?
I tested this on versions 5.0, 5.2, and 5.4, and the results were all the same.
My operating system is Windows. I also attempted the same process on a different computer (Windows) within the same network, and the results were consistent.
But it works on same network different computer in MAC OS.
Yes, we discussed this a bit on the developer call yesterday and the consensus was that there is an probably and incompatibility with some proxy being used on your network. Unfortunately it’s not clear what if anything we could do as a general solution for this. If you have a chance to research this with your local network experts and suggest workarounds that would be appeciated.