Gives error 426. (upgrade required)
Looks like after all the LAN mode was not really LAN mode and both devices supposed to be completely banned from accessing the internet.
Both devices are banned from upgrading due to company policy (Bambu Lab failed number of background checks for IT security/procedures).
What now ? Did Bambu Lab bricked both X1C ???
Where does it state that Error 426 equals “upgrade required”? Upgrade what, your nozzle?! 
Even if it was an update, i am sure the machine would tell you that instead of that common FTP error due to your firewall settings.
There is nothing bricked there! You blocked, as you said, your printer and that’s it. Some routers block even lan communication.
Remove that block and you will see that everything works.
If you want a better way of doing it, put you X1C under parental controls in your router and define the time to never be allowed to use the internet. Done!
Note: we need a Facepalm emoji… Pretty please 
I would test with new SD? And also ignore comments like the one above 
In FTP 426 usually means data transfer was aborted due unspecified reason before data transfer is completed or a connection issue. My understanding is that FTP should work normally in LAN-mode, at least in latest stable FW.
? It worked for 1 year fine, now getting on both same error, network is fine, same with routers. It will send files but won’t start the job!
THINK before you post such post. 
That is connected to enterprise network, no such thing as parental control LOL.
It worked fine on both units Yesterday, Today both are dead and unless sent file to printer, they refuse to print anything from Bambu Studio.
The HTTP 426 Upgrade Required client error response status code indicates that the server refused to perform the request using the current protocol but might be willing to do so after the client upgrades to a different protocol.
On both units ?
Sending files works fine. It won’t start print only on both units. No matter reset/re-pairing
He should not ignore when someone tells him the obvious. There is no change (yet) on the way that LAN mode works.
I will reiterate what i have said before.
“When these kind of problems exist, usually the problem is between the chair and the device”
Mongering about this as if Bambu had changed anything right now is just plain dumb. I am not on Bambu’s side, hell no! But this is an error that has even been discussed in the Github. Sometimes it happens, sometimes it doesn’t.
I can bet that the problem is in the router side. Removing that block and applying the rule i gave above should fix it.
Some people are just obsessed right now with this bambu drama and that is not good.
You do you, i’ll keep doing me and i stand my point.
I am out of this one. Good luck
Just curious will it print from the SD Card?
I’m assuming these are printers that have been working with the “internet ban” before and no network changes (router, firewall etc) have been made? You are mentioning “company”, therefor the assumptions.
I’m not finding any hint to “current drama”, if software and network-usage on these devices do not comply with OP’s company’s policies they don’t get to access internet, that’s it.
In order to have better possibilities for community to help OP please post firmware versions and Bambu Studio version, are you using Bambu cloud (most likely not?) etc any other information that could be useful.
I can send files to both units and printing is working fine, I cannot only start print directly from bambu studio.
I will add that I am using older version bambu studio due to number of highly calibrated commercial products (new version were causing havoc and with > 200 designs I wouldn’t invest time to re-calibrate them), same with FW.
I have stopped upgrading after the Bambu Studio was put on a black list and I was banned from using their cloud (at least according to the memo).
Bambu studio is v1.10.1.50
FW is 01.07.02.00
Yes that is correct, both were working fine at the end of working day.
Today I cannot start print directly, but can send it and start print manually.
Both using same settings, no changes were made to them in couple months at least. They were just printing the same stuff.
Micro sd cards both are top quality U2 from Sandisk (extreme if I remember well), both confirmed to be originals.
When printers are banned from internet is it possible their time is wrong and connection would act up? Are printers showing correct date and time? (They use hard-coded public NTP-servers)
Ok based on other people investigation.
Bambu won’t allow me to print unless I update the FW/Bambu Studio.
Yes it is connected with recent drama. So it looks like they indeed ban you from using printer in intended and advertised way.
===update===
Without going online and updating it won’t work.
As soon I have connected via non-coprorate network to the cloud I have downloaded update and it works again… However I am dissapointed that they had a backdoor that was able to stop me from using lan only mode forever (I haven’t used it in cloud for months on old version).
Export the .stl and open it in a new window. It’s a problem with the 3mf. I dont know why, but I’ve had this problem with a few models as well.
I just found this thread because I was having the same problem. It wasn’t even as difficult as @F1R3CH1CK3N suggests . . . I was able to just move a part around on the build plate, slice, and successfully send over LAN.
Oh dang lucky! It was kind of annoying for me to move them to a new project to print. It was a big project that had quite a few adjustments with painted supports and stuff. I wonder what the real problem is. Ill try what you said also!
You are hugely wrong. It has nothing to do with bambu forcing you to update firmware or go online. Please stop spreading misinformation.
Logging in to confirm I had the same problem and, despite the error text pointing to an FTP protocol issue, leading me down the same troubleshooting path as @carmamir , it’s misleading.
Moving the object slightly on the plate to re-slice, as @palutke mentioned, resulted in it sending and printing fine. Very weird!
Sorry to bump an old thread (again) but just wanted to +1 this for anyone else coming across this: HTTP 426 ≠FTP 426! I’m still not sure what causes this (maybe transient network issues?), but assuming 426 = Upgrade Required is a red herring AFAIK.
