After a change of gearbox my mechanic is no more able to start the engine.

If we try to start there is a very strange sound from starter and Error "F" on the Display.
Engine not turning at all.
We tried with different starter. Battery is perfect. Flywheel is turning.
We have the original Software for Update via ODB and tried to reset and calibrate all.
We tried several days now and have no more power...
I cannot post a video of the starting process but when turning key it just make a "Krk" Sound and
engine is not turning at all.

Here short movie of the starting prociess:

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Sorry I can't add much except that something is def not right
perhaps something put back wrong - not aligned, missing, loose, dropped part, etc
preventing things from turning maybe

accidents happen
but this is 100% on your mechanic to make right
Where was the work done?

 

That sounds horrific!

 

Sounds like the starter is not engaging properly into the flywheel. How do you know the flywheel is turning for a fact? Is the flywheel bolted to the engine?