[Rey_a]

wie vorgeschlagen, habe ich ein Backup erstellt, MSP deinstalliert, das Tablet neu gestartet, MSP neu installiert und dann den Test "Alle Teile" gemacht. Es wurden Daten übertragen.

Ok, if I understand you correctly, after reinstalling MSP, you did see midi messages appearing in the MSPro screen on the tablet when playing notes on the keyboard.

Which would indicate that MIDI communication was workiing at that time!!!

And then you restored the backup. And after that things stopped working. 
A question, when restoring from the backup, did you also restore the MSPro settings from the backup? (this is an option in the restore dialog) ??
Because if you did restore the settings, there may be a setting restored that causes the problem. In which case I would advise to remove MSP again, reinstall it again and then restore the backup WITHOUT restoring the settings (see image). And then try the ALL PARTS test again.

Rudy