Ping to bridge internal host works for about 10 seconds

[RevTerr]

Greetings,
I have this very weird issue on VirtualBox 6.1.32
I have a virtual host (let's call it Jupiter) on Windows Server 2019 Datacenter, with two network adapters, both bridged:

• -an "external" IP, in the 85.12x.x.x public range
  -an "internal" IP, 10.10.0.29 (subnet mask 255.255.240.0 -I know, plenty of IPs)

I also have a couple of virtual machines:

• -let's call one Ganimede (Windows Server 2019 Datacenter), using two bridged adapters:
  • -an external one bridge-linked from the external one on Jupiter, again with an IP in the public range 85.12x.x.x
    -an internal one bridge-linked from the internal one on Jupiter, with the IP 10.10.0.9
  -let's call the second one Europa (Centos 7), with a similar layout of bridged adapters:
  • -an external one bridge linked to the external one on Jupiter, again with an IP in the public range 85.12x.x.x
    -an internal one bridge linked to the internal one on Jupiter, with the IP 10.10.0.113

Both these Virtual Machines have websites on them (a Sharepoint and, respectively, a Moodle) and both can be accessed from "outside", via the external conection (85.12x.x.x). However, the websites cannot be accesed from the internal network (10.10.0.x). The browser on computers claims websites took too long to reply, ping from anywhere inside 10.10.0.x claims Destination host unreachable

EXCEPT (and this drives me mad!):
If

• -I change the settings on Jupiter's internal connection (enable DHCP, for instance, or conecting it to the internal gateway, or vice-versa)
  -then I briefly disable the internal connections to the two virtual machines
  -then I reenable it

for around 10 to 20 seconds everything works; the websites are accesible, ping responds properly... everything works. And then again everything stops.

I tried disabling the antiviruses and firewalls, I changed the network switches along the network... no go. At this point I'm looking at reverting to a previous Virtual Box version just to try something, because this layout used to work before the pandemy (we have just returned to on-site activity)
Any ideas?

[fth0]

Since you have two possible network paths between any pair of IP hosts, you should perhaps investigate the routing tables, especially look for the metrics (route precedence).