Seem you have newest meter, compared all stuff we have here. Would be really interesting to see which Rev is yours?
Sometimes many errors are gone.
Is it after warmup? I mean do you have selftest errors if just power-on cold meter, but they gone after it run 30 or so mins?
It can be dirt on PCB, which conducts current somewhere on analog board, but I'd hold off cleaning for now, as improper cleaning
can make it totally worse, as analog board is very sensitive for leakage. I have one PCB which have random errors, and plan to
debug it next week, so maybe then can help you for more specific diagnostics.
The cal seal is gone is this a problem or can the cal button be pushed?
No problem, cal button just unlocks calibration mode in main menu (
Comprehensive calibration, manual PDF), in which meter will ask
you to provide precision voltages and resistances to it's input to perform self-cal.
I would not suggest doing so, unless your source is Fluke 5720A or similar grade calibration source.
If you send meter to calibration lab, they don't care if there is seal present or not

Tomorrow I'll send one of my K2001's (which i got pass self-test anytime everytime if powered on 110VAC mains) and K2400, will see
how it goes...
On chinese bbs some guy posted docprint corner of K2001 schematics, it's marked Rev R.
While mine analog boards are Rev L and Rev J.
You can find revision letter in analog board corner, near rear inputs fuse.
Mine: 2001-102-04
L. 04 means layer, it's 4-layer PCB, so it's marked on every layer.

I must say I can hardly work on this project due to a lack of time so it can be that I will respond with a delay
No problems, take your time. Even while quick response is cool, we all here sharing just for fun, we still have stuff and work to do

Interesting note:
I did some testing with variable input voltages (mains) using Chroma 61504 AC source (2KVA)
AC input settings: 90VAC, 132VAC, 180VAC, 250VAC - 50Hz - meter failed self-test with 200.1,200.6,200.7,201.2 (but still able to do measurements)
AC input settings: 90VAC, 132VAC, 180VAC, 250VAC - 60Hz - meter pass self-test without any errors, on all input voltages.
AC input settings: 90VAC, 132VAC, 180VAC - 400Hz - meter failed self-test with 200.1,200.2,200.6,200.7,201.2,401.1 (but still able to do measurements)
AC input settings: 250VAC - 400Hz - meter failed self-test with 200.1,200.6,200.7,201.2 (but still able to do measurements)
Something is screwy on that auto-selection input circuitry maybe...