client is created, but response still is 406

we are using json and also tried other formats, but no success so far
config = {
...
:format => :json
}

ActiveResource::ClientError: Failed. Response code = 406. Response message = Not Acceptable.

https://.../api/clients?filter%5Bnumber%5D=159857&format=json

site must be https://my_sub.salesking.eu

otherwise it did not work for us. (ree-1.8.7)

  def set_api_connection
    SKApi::Resources::Base.set_connection({
      :site     => 'my_sub.salesking.eu',
       ..... 
       })
  end

when we search by email/number we only want the first result, no need to fetch and parse 100 records <-> support &per_page=1

it would be great if simple stuff like save could also work in 2.3, the generated http requests should not be that different (we just tried and requests look roughly the same) only problem is that we are getting http 500 errors from salesking

Could be automatically set to subdomain,
in your example organisation was missing

we used this:
organisation = SKApi::Resources::Base.connection.site.host.split('.').first

client = SKApi::Resources::Client.new(
  :organisation => organisation,
  ....
)

without a local salesking installation, please provide a test account live/staging etc

Cheers
Mark