Issue with kotlinx/serialization in spring boot

I am getting this error in a spring boot web application. It’s driving me crazy.

I am trying to move to the newest spring boot version (2.6.5).
kotlinx-serialization-json is not a dependency.

Does anyone know what the problem is and how to fix it?

Caused by: org.springframework.beans.BeanInstantiationException:
Failed to instantiate [org.springframework.boot.autoconfigure.http.HttpMessageConverters]: Factory method 'messageConverters' threw exception; nested exception is java.lang.VerifyError: Bad type on operand stack
Exception Details:
    org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverter.<init>()V @8: invokespecial
    Type 'kotlinx/serialization/json/Json$Default' (current frame, stack[1]) is not assignable to 'kotlinx/serialization/json/Json'
  Current Frame:
    bci: @8
    flags: { flagThisUninit }
    locals: { uninitializedThis, '[Z' }
    stack: { uninitializedThis, 'kotlinx/serialization/json/Json$Default' }