The sketch I provided you with because of the complication problem is being compiled right now.
But running it leads to a less expected behaviour.
It is not stable at all. Where I do have some odd ten units working quite OK compiled with version 10.4 it now breaks completely.
The statement causing the problem is in the setup:
When I comment this statement out, the builtin LED is blinking as expected.
Uncommenting leads to crash.
No clue what is going on.