Yup. Ran that and variants like it many times. It would say something along the lines of: 0 updates available 0 updates installed 2 not updated. I couldn't find out why it wouldn't upgrade it.
I tried:
"sudo apt update
sudo apt full-upgrade"
Then I tried:
"sudo apt purge --autoremove raspi-firmware
sudo apt update
sudo apt upgrade"
Gave me another error.
Then I went went into Synaptic, I reloaded "linuxcnc", marked the new version for upgrade, and then clicked apply. It gave me yet another error.
During a cursory search of the internet and that's when I learned that it's most likely due to an outdated repository link.
Then I asked here about finding a good repository address.
In the meanwhile I tried several different addresses from internet searches, tried them and all gave me errors.
Check back, found the post was moved and had no replies.
I used balena etcher, backed up the entire drive image just in case what I wanted to try didn't work, made copies of the INI and HAL files and their respective backups, loaded up a fresh linuxcnc to a bootable drive, used the installation to wipe the partition and installed it fresh.
Other than the new version being picky about what letters were capitalized in the name of the HAL file, and a couple of minor tweaks in Debian including reinstalling the Etouch driver so the touchscreen drivers would install, everything seems to be working fine now.

I'll swap them both and see if that solves the problem. Otherwise, it'll be difficult.

That makes sense, the 75175 is likely the differential encoder input circuit
and the 75174 is used for an emulated encoder output or some such...

 

I traced the path on the circuit board a bit and came across the following two components: 75174N-H and SN75175N.

Unfortunately, during testing yesterday, we blew out one of the outputs.

Ouch!

A bit late, but one trick is when testing, to have a say 470 Ohm resistor in series with the +24V used for
sensing the output, that way the current is limited to ~50 mA in the event of a wrong connection.

What chip does the encoder input use? If it's differential it may be something common like a 26LS32

 

Thank you very much for your help. Your description provided the solution!
Unfortunately, during testing yesterday, we blew out one of the outputs. I have already ordered a new optocoupler. I also have a second inverter, but it has a fault at the encoder input, as the signal is not being detected. I installed this circuit board and on that board, the output switches correctly as well.

Perhaps someone here is familiar with the Yaskawa board. There must be a defective component somewhere on the board preventing the encoder signal from being processed. The encoder input is connector 2CN.

Some more info.

Stell direkt eine Anfrage der Support ist sehr hilfreich.
Und relays:
Ich würde da was von Pilz nehmen 

I got completely turned around looking at those pics the first time.  I thought the surface you were resting the belt on was actually the pulley teeth in mesh with the belt.

Duh.

The dark line in the middle of the tooth is just the last bit that still has the original surface, with a bit of dirt that makes it darker. On both sides, it is mostly obliterated. I assume that is what you mean by “tit.”

And I think the gap between the teeth is also to reduce the smallest possible bending angle, in addition to the reasons you listed.

Is that supposed to be a T5 belt profile?  I've never seen a timing pulley with a 'tit' on the tooth crest.

Is the tit on the pulley teeth from wear, or is it some substance that got in the belt tooth valley groove and stuck to the teeth? And that's what you're cleaning?

To clarify, here's a T5 belt profile from Misumi.  I think the groove is there to help air escape at higher speeds (noise reduction).

 

Bei Amazon hätte ich nicht geschaut image missing /media/kunena/emoticons/laughing.png. 

Motor, gefunden bei amazon. (Bei Stepperonline sind sie gerade nicht lieferbar.) 
Encoderkabel mit Batteriekasten, bei stepperonline, sonst nicht gefunden. 
Netzfilter FN2090-8-6, bei ehbshop.de
Das mit dem Relais bereitet mir noch Probleme. Ich weiß nicht, welches ich genau benötige. 

I finally found the problem with the threading. Should have found it much earlier.It’s the belt.

It gets chewed up. Opening the side panel, I found belt-colored dust, but the belt still locked fine, at least from the side. Looking closer showed that the teeth are broken up from the middle outward (see picture).
I found one tooth gap with some hard thing in it, so I assume that my initial cleaning when installing the belt was not very thorough and I overlooked that.
After cleaning that out and reinstalling the already damaged belt, I could thread fine. But I think this will not last long, so I ordered new belts and will install them after VERY thoroughly cleaning both pulleys.
It’s really typical for me that the solution to a month of problems is: “You should have cleaned the thing better.”

Wenn du bei Amazon bestellst, geht's schnell aber kein STO.
Wenn du direkt bei Stepperonline bestellst, freu dich auf den Zoll.