FYI. This was a fix for my SLK R171 but since it shares the same EIS as W211 I hope it helps you guys as well.

detailed steps are found here
https://www.slkworld.com/threads/eis-internal-switch-cleaning-quick-version-fixes-morning-no-crank-problems.592049/#post-4897245

Definition of “Morning no crank”
1. Key can enter the key hole and you can turn the key to position 1 and 2
2. Car battery is good
3. You hear the steering lock disengage and you can turn the steering wheel. (quick whirring sound)
4. Gear is in P or N and its indicated as usual on te dash
5. Accessories light up as usual.*
6. Turn the key to position 3 to crank the engine but nothing happens. no clicking sound, no dimming lights, its as if you never turned the key
7. No error codes raised.
8. You try again, maybe a few times, suddenly the car starts as normal.
9. Car will restart easily for the rest of the day
10. The problem is sporadic, it might not happen again till next winter or it can come back next week. But over the course of months or even years you notice it is happening more often.

condition 5 is optional, some accessories might not light up (depends which internal switch is intermittent). Condition 10 is very important. This rules out a bad starter relay. Condition 4 rules out a bad shifter immobilizing the start sequence. Condition 3 rules out a bad ESL. Note its important that steering wheel disengages, hearing the whirring sound is not enough, the lever can still be stuck and start sequence halted.

The problem is caused by switches inside the EIS whose contacts have become intermittently contacting. Cleaning them will solve the problem. An analysis of wether this can be considered a permanent fix is not treated here. It will be in the next DIY.

steps overview:
1. Remove EIS
2. Open EIS
3. Remove key tumbler from barrel and clean all switches inside barrel.
4. Reassembly (This is not a direct reversal of dissassembly, steps are different.
Each step will be detailed in a separate post.
tools
you will need
1. Torx screwdriver
2. Soldering iron
3. Desoldering tool (cheapo plastic one is good enough).
4. Solder wick. (this is a must, if you don't know what this is, its best you do not proceed).
5. Solder.
6. Contact cleaner. Get a full can, get the best you can buy
7. Hair dryer
8. Thin wire, awg 22 or smaller. Kynar wire wrap recommended.
9. Small screwdriver for poking and prying things open or similar tool.

Skill level:
You must be moderately experienced with desoldering circuit board components.

Risk:
If you are experienced in electronics and you lift a trace while desoldering, and you know you can repair it with a jumper wire, risk is very very low.
If you are a newbie and don‘t know what I just said, do not proceed.

Idea:
if you’re not adept at desoldering things take your EIS to a phone repair shop. They are experts at this!

This is an awesome repair - Thank you for sharing it here. (W212 has a similarly amazing EIS).

As you pointed out: NO CODE... good luck getting that resolved

Here is the rest of the EIS teardown
https://www.slkworld.com/threads/eve.../#post-4905196