Today I got an email from Teenage Engineering Support! They wrote:
"After reviewing the issue with this OP-1 we’ve come to the conclusion that the units DSP board is acting up,
we are currently out of stock on DSP boards which means that we won’t be able to repair this unit until we’re restocked
in late March to early April.
We will contact you as soon as we are ready to receive your unit.
We apologise for this and hope you understand."
Thank you TE-team, I think my problem could be solved by this.