Summary

Our customer operates a small shop on Odoo 15EE on odoo.sh. The problem concerns PoS: The PC that should run PoS ("PC1") can not reliably access the IoT box, i.e. it can't print receipts and the customer display only shows the set-up screen ("the customer cart will be displayed here once a Point of Sale session is started").

The problem in details

Access from the PCs to the IoT-Box is unreliable. That means: for about a day at a time, we get one of the following three behaviours, but we don't know how this decides itself. It seems to be related to rebooting the IoT box, but we are not even sure of that.

Things worked OK for at least a year or so, then we tried to upgrade the IoT Box from the present 21.04 to 21.10. At first, everything worked, The problem started to occur with increasing frequency on 21.10, so we stepped back to 21.04. There, we saw mostly "PC2 works, but not the others" pretty stable, and it turned into "nothing works" today. 

• On some days, only PC2 has access to the IoT-Box.
  If the Odoo App "PoS" is started on PC2, it can connect to the IoT-Box, the corresponding icons are green, the customer display shows the shopping cart and the receipt printer prints receipts.
  But the same App running on PC1 or PC3 can not connect to the IoT-Box. The icons are red, every operation leads to a pop-up "can't access IoT-Box", receipt printer doesn't print and customer display only shows the "the customer cart will be displayed here once a Point of Sale session is started" screen. 
  At the same time, all three PCs can access the IoT Box set-up screen.
  It is particularly annoying that lately, it always "works" from PC2 which is *not* in the shop, so the customer is making a remote session from PC1 to PC2 to get as close as possible to normal operations.
  In the Odoo IoT app, everything looks OK. The IoT Box shows up and the devices  seem to be connected. 
• NO PC has access to the IoT Box: PC1 and PC3 behave as above - and PC2, too: error pop-ups, no receipt printer, "no" customer display.
  All three PCs can access the IoT Box set-up screen.
  In the Odoo IoT app, everything looks OK. The IoT Box shows up and the devices  seem to be connected. 
• ALL PCs have access to the IoT Box, can print, feed the customer display... come to think of it: until the IoTBox upgrade, we know it worked from PC1 - but I don't think the customer tried PC2 or PC3. So we may well have had the same problems with the other PCs but we didn't realize. 
  

Network set-up

There are three "regular PCs" in the network, let's call them PC1, PC2 and PC3, and the IoT-Box. All of them are connected via Ethernet and supplied with their network information via DHCP. All relevant devices (IoT-Box, PC1, PC2, PC3) always get assigned the same IPv4 address. IPv4 and IPv6 co-exist in the network. 
PC1 is "the PoS station". It is located in the shop side-by-side to the Pi, the receipt printer, the cash drawer and the customer display. 
PC2 and PC3 are in the backoffice.

The IT infrastructure guy and I have tried everything we could think of (and we have more than 40 years of combined IT experience). We are after this for weeks now but we can't find any anything that explains why the PCs behave differently. IP addresses (at least IPv4), routes, firewall rules (IoT Box and LAN)... no explanation. What is most puzzling:
All PCs have access to the IoT Box status screen from the browser always (http, https via nginx proxy, but also port 8069 direct).
The IoT Box seems to always register correctly with the odoo.sh "IoT Box" app.

Any ideas?