ARP odyssey 2823 cv problem

Strangely my VCO1 is not following the pitch of the keyboard (where VCO2 does) … any idea ?

I suppose you read this.

ARPS are like a violin, you need to keep fiddling with them to make sure they work.
If its not a keyboard connection issue look for a bad solder joint or broken wire.
After that its a failed component, Contact your local music store to find out who has the know how to service them.

Synthchaser is the best source for repairing / or supplying newly made replacement parts for ARPS.