Here's, what I got out:
First I checked the voltage coming from the ECU at the 5-pin-male-plug (ignition ON, engine not runing, transmission in park):
blue / orange / green / purple: 12.2 volts (battery power)
tan: 0 volts
Ohms at 5-pin-female at transmission:
blue: 2.0
orange: 1.5
green: 1.3
tan: 0
purple: 0

I'm wondering that there's no 12+ volts on the tan wire. Put the transmission also in overdrive, ignition ON, engine not runing, but that didn't changed anything.
Does it mean my ECU could be bad? - Should I search tan on the ECU and check for 12+ volts, to ensure the wiring harness isn't the issue?
What about the purple wire? - 12+ volts from ECU, but no ohms reading at the transmission.

What is the resistance of the TCC solenoid?

Along the the wiring diagram the TCC-solenoid has the blk/tan wire. There's no resistance and with key in position 1 (engine not runing) shift stick in park or overdrive, there's also no voltage from ECU at the plug.
That's the reason while I'm asking, if there should be a voltage (e.g. 12 volt) like the other wires going to the plug.

So it read ZERO ohms or open circuit? You don't read resistance with power on. You read it's resistance at the female. The solenoid is inside the trans.

https://cimg3.ibsrv.net/gimg/www.gmf...78e71927f2.jpg
That's the ohms reading from the TCC-solenoid.

Yes, the solenoid is still inside the transmission. The main question at the moment is: When is 12 volts delivered by the ECU to TCC-solenoid? - With key in position 1, or mabey at a speed of e.g. 50 miles given by the speed-sensor, or mabey when 4th gear is activated, or if shift lever is set to overdrive.

The TCC is not commanded under 43 mph. How do you have the leads hooked up to get you reading?
1 ohm is a near dead short.

I put one lead to ground and one to the plug at the transmission. installed
On Saturday, I tried to find out what's going on: I had a brand new out of the box ECU in stock and put it on (but had to use the prom from the old ECU). I went on the road for a short test drive, but nothing changed. I read again the codes and got Code #26 #39 #48 #64 #65, so it couldn't be a faulty ECU.

I found someone, who posted the same issue and he could solve it with a new EGR-Valve. I can't understand, how an EGR-Valve can prevent to lock up the torque converter, but I had a good used EGR-Valve and it just take a minute to install it. Did a test drive an the issue seemed to be solved.
From Saturday afternoon until today in the afternoon, I drone about 200 miles and the torque converter locked up every time as usual. Today in the everything the issue was back and also the trouble codes.

I guess, there isn't any fault at the transmission itself, it's just the part that get impared, also the ECU is just throwing codes like wild and mabey we can't trust (all) the codes.

I'm not sure if the car need a mechanic or an exorcist 😉.

LOL you might need one of each!

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But seriously, what codes are stored now?

When it has codes stored for some (maybe all?) emissions-related issues, it will refuse to lock the converter clutch. I've seen this on many OBD1 GM vehicles from the early `80s all the way up to OBD2 taking over.

What are the codes now?
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You have a good sense of humor in it all.