Networking issues

[TribbleSmasher]

My guess is the most modern macOs is looking to connect to the other computers based on modern security protocols and this must fail as you probably never upgraded the Winxp machine or the Amiga since years ago. The macOs has certainly updated everything required.
If you didn't change the physical components of the network (new router? different cabling?) only the software side remains.

You might be able to downgrade the Macs firewall etc to allow legacy networking.
If you got a new router then there might be network settings as well you need to take care off.

Another option is to try and add a simple NAS to your network with open unrestricted access just for sharing files.

[TribbleSmasher]

It is a Network Attached Storage. In the most easy case some routers allow e.g. an USB flash drive plugged in  working as said storage. They need some configuration in the routers setup, which way they are secured and accessible and what not.

[TribbleSmasher]

You are the last one trying to help so we'll blame you. D)

[TribbleSmasher]

Winxp sometimes needs a couple of minutes, or a restart, to recognize network changes.