IIRC the Digis use endless potentiometers, not encoders. For encoders, you don’t want to rub the grease off, afaik it protects the encoder from dust, particles etc.
Maybe someone can confirm that they are indeed potentiometers, before you continue your cleaning efforts.
Edit, seems to be the case:
There’s also cleaning advice in the thread. Personally, I’ve no experience with what works best, but you can always contact Elektron support and ask them what they’d recommend.
You could go into the internal test mode (hold function while powering up to bring up the startup menu, then press trig 1 to enter test mode) which lets you test various hardware components as well as the software side - if you want to rule out any software bugs…