If any of you fellow builders encounter problems and error messages during compilation of a previously downloaded firmware file please download the freshest file for a fix.
Some of the previous code had a remark line on top of the code which by mistake were left uncommented and thus the compiler tried to interprete it as garbage…
It seems that only the last revision of the nanoKeyer preconfigured REV C firmware was affected.
For newly built nanoKeyers I would always strongly recommend to download the most freshest file prior to uploadig to the Arduino Nano.
I have just uploaded a fixed version of the REV C firmware file. It also contains a fix for the previously described speed potentiometer default configuration problem when uploading to a factory new Arduino Nano.