Hello all, first of all, I want to thank you all for the ideas and support. Never found a positive community like this. So, thank you again.
A couple of things: setting a VPN to manage two printers is a little overkill, I think. And the first MFP is fairly recent, the second one is an old Xerox w/o any advanced features.
Yesterday evening I was a little tired, so I saw a movie and ran into Morpheus’s arms.
This morning I’ve tried to set a static route on the “main router” as suggested. The router is a Chinese box, can’t permit a static route. Then, I put in the middle a strong, old and heavvy Zywall 5 I bought long time ago (and payed with a lot of blood). And now (don’t know why) everything works like a charm! In effect, I haven’t changed anything in the network, only installed a better router in LAN1, but in the LAN1 side it’s used only as a switch! Can’t believe this.
Now, the first step is complete. After this, I have to shape the IP traffic, share the printers, set the privileged users and finally test everything. Just some days of work 
really glad to have been here. To the next. Bye
Why not put NethServer as router? It’s far better than Zywall 5.
Had problems around 1903 with Win10 for example… (Not quite sure anymore if just before or just after). MS made a big boo boo with 1903, like forgetting to include the Repo with MediaPlayer (Included in Windows for ages.). The 32Bit Version is necessary for business applications like dictation - they use the included libraries.
MS cleaned that up within two weeks, but MediaPlayer was ready for use before that update, and that update simply removed that program. I had to wait 2 weeks until all 5 new PCs got the next updates and the fix for that.
My 2 cents
Andy
OK, that’s sadly “typical Windows update doesn’t provide XY stuff anymore” but how did that affect routing?
Routing failed around then, SNMP too. Most due to the fact that all settings were gone, or had to be reset (eg deactivated, reactivate)…
Worked shortly afterwards again, but if due to additional updates or reacivation, I’m not sure…
No one really uses the 32 bit Media Player anymore. But - like a lot of 32-Bit stuff - it’s needed for “Integrations”. Like Dictation with Philips Systems, they make use of the 32-Bit DLLs.
Why else does Microsoft still Provide a 32-Bit Version of office?
You can’t buy a 32 Bit box for several years now…
But there are still a lot of companies out there using legacy crap like VB for office stuff.
I fully agree - they should upgrade their code from the soon dead 32-Bit VB…
But to recite from The charge of the Light Brigade (Crimean war over 100 years ago, british…):
Their’s not to question why
theirs but to do and die.
My 2 cents
Andy
A media player crashes routing…that’s really weird.
It wasn’t the media player, there were a LOT of bugs released with the initial 1903 Upgrade.
1903 was a “biggie”.
They did release a lot of corrections very soon afterwards, check your own updates at the time…
Andy
I had 5 new Win10 PCs, monitoring with Zabbix set up and working, with both SNMP and Zabbix agents.
After 1903, SNMP didn’t work. The Agent and Location entries were still there, but any IPs entered in were gone, as well as traps like “public”.
And SNMP wasn’t available to install/reinstall…
Took 1-2 updates to get it working / available again…
OT mode on…
@Andy_Wismer don’t forget that every version of Windows 10 is kinda like the “not LTS version” of Ubuntu…
Only few of them are long-term supported. And sometimes win some of… new features.
OT mode off.
It’s not a bug, it’s a feature!
As Cockpit?
Couldn’t help smiling…
But there are a lot of stuff which works very well!
Also With Windows 
Also here, fully agree!
I AM writing this on my Macbook, but I’ll admit that my Win10 Parallels is also up and running…
There are reasons for everything - not all are sensible, but still!
@Pike You’re right. But remember it’s a test config. The network here it’s “a little more complicated” than I told. There is a “main router” by the internet provider, with wifi. A mini router (the one Chinese) acting as a wifi client for a subnet, now the Zywall 5 and The Nethserver. In office the network is less complex. Without the Zywall, don’t know why, I had the problems told before.
Hello everybody. It’s time to make amends.
This morning I have had the same issue I had days ago, the reason why I opened this thread. So I thought “there’s something strange in this behaviour (who can you call? Ghostbusters!) 
”
Well, now I realize that I’m stupid. Never, never felt so stupid.
In the windows 10 pc, I deactivated the windows firewall, but the antivirus, AVG internet security, has its own firewall, that I forgot. I replaced the windows pc with a xubuntu one and Eureka, all works flawlessly. No need of Zywall 5, no need of magics.
Nethserver always worked perfectly. I never, never, never felt so stupid.
There’s always something to learn, and to remember. Next time I’ll not make others to waste their time.
Now you can think that some italians are stupid, surely at least one.
Again, thank you. Bye, Mauro.