Hey Boris!

You disappeared for weeks. I was worried that you were conscripted and sent to the front line in the temporarily occupied Ukraine. I'm glad you're OK.

Now you indicate another clue, the shift lock fails with the lights, eh?
Relay 2/26 is common to the lights and shift lock, both are switched by the ignition switch.

And you checked all the fuses and relays and none are intermittent or have
a loose connection. You can have relays go intermittent, and also
relay socket pins can get loose, usually after several removals by enthusiastic brutes that can rock the relay and damage the socket pins.

It's a good idea to take a 1/4" male spade connector and crimp it on a solid
copper wire so you can test each relay socket pin to see if it is tight.
If a pin is loose then it needs to be removed and tightened or replaced.
If you get to that point I'll tell you how to do it.
Use Penetrox-A, anti corrosion zinc paste, on female connectors.
Volvo had a connector grease Part # 1161417-9 if you can't find Pentrox-A.

Here's what to do: The next time it fails be ready with your VOM to measure
+12V at the top of Fuse 26 with the Key remaining in KPII run position.
Look carefully at the top of your ATC fuses, you can probe each leg of the fuses. Ground your meter at the Negative battery terminal.

No +12 Volts then it's a bad relay 2/26 marked "106", or the ignition switch.

Remove 2/26 and measure +12V at pin #2 with your ground lead on pin #4
If you get +12 your ignition switch and ground 31/50 is OK.
measure +12V at pin 1 to pin 4. If that's OK try another relay

If both of the above tests get no +12V, try the test again except ground your meter to the Negative of the battery, and if both are +12V check for a crappy ground 31/50 which is under the main fuse box, lower level, in the engine bay.

Use conductive grease nickel or copper anti seize or Penetrox after cleaning up the connections.

Report back what you find, Bill