Mindflex > Arduino > laptop over USB > Processing > Pd
The Arduino Brain library just passes the serial data, so also sends errors.
Those errors messed up the packages arriving in the Pd/Bela patch.
By writing a custom processing code, the errors are now ignored and not send to Pd so the packages are received well.
It's quite a digital detour, but another advantage is that Processing code now also gives me visual feedback in the form of growing triangles over 8 different brainwaves what makes (much needed) brain training easier.