Software update wants to download updates from my local network

For a few weeks, macOS software updates have been consistently failing. The download bar does not make any progress and I get various error messages such as "update failed" or "failed to connect to server".

However, the issue goes away when connecting to a different Ethernet network. This being said, I would really like software update to work with my home network.

Today I looked into /var/log/install.log and I realized that Software Update is trying to download the updates not from Apple (swcdn.apple.com) but from two hosts on my local network (non-routable IPs, e.g. 192.168.0.3)...

If I run AssetCacheLocatorUtil I get something like this:

Found 2 content caches
192.168.0.3:55125, rank 1, guid xxx, valid until 2021-09-20 16:49:48; supports personal caching: yes, and import: yes, shared caching: yes
192.168.0.7:49707, rank 1, guid yyy, valid until 2021-09-20 16:49:48; supports personal caching: yes, and import: yes, shared caching: yes
Determining configured public IP address ranges...
No public IP address ranges are configured.
Testing all found content caches for reachability...
This computer is unable to reach 2 of the above content caches:  192.168.0.3:55125, 192.168.0.7:49707

One of these IPs is that of a Mac laptop, the other one I don't know, maybe it was another Mac laptop ages ago, but it has not been up recently. Anyway, these are family laptops, and they have never run macOS server nor have had any server or caching options enabled.

So how can I make macOS forget about these local IPs and use only Apple's servers for software update?

Bonus points if I can understand how this crazy config came to be.

I already tried this, without any success:

• delete /Library/Preferences/com.apple.AssetCache.plist
• delete /Library/Application Support/Apple/AssetCache
• and then run sudo AssetCacheManagerUtil reloadSettings

Note: I'm still using Mojave, trying to upgrade to Big Sur.