I'd have a look at your host OS firewall settings and the guest one too.
With Windows XPMode the latter is found under the Control Panel > Security Center. You should see that the AV and Firewall are activated. Assuming you are using Windows firewall and not a third party one on the VM, at the bottom of that page you have a link to the firewall configuration menu.
Click on that and then click on the Exceptions tab, which lists.........................exceptions.
I'd compare it to the other PC's XPMode VM settings and if there are any differences that could be the problem.
This is a long shot: I'd also have a look at the network settings being used by the VM.
I've not had it with a VM myself but at one time on my host PC my ISP's automatic DNS server of choice started blocking Malwarebytes updates. However when using the XPMode VM on which I also had Malwarebytes installed it updated on the VM without issue.
I thought that odd and compared the server settings and found (I'd forgotten) that I'd set it up to be exactly like my old XP laptop that used a specific DNS server not the automatic one.
I did not want to mess that up by testing it by swapping it to automatic and as I'd already updated Malwarebytes I thought the sensible thing was to swap the host PC to using an alternative manually set DNS server instead too.
When I did that the update problem disappeared.
I should say that since then whatever the problem was has been fixed and the ISP's automatically selected DNS server does not interfere with Malewarebytes updates any more.
