Trying to complete the Challenge in the Setting up Lightning Connect module and am getting the following error:

Challenge Not yet complete... here's what's wrong:
There was an unexpected error in your org which is preventing this assessment check from completing: System.ExternalObjectException: data.api.DataSourceException: This data is no longer available. The "phone_plan" table in the external data source is currently unavailable. Try again later or contact your administrator for help

Not sure what the deal is.  Please advise.

Hi everyone,
I am now trying to build an OData 4 service so that Salesforce can access the external data.
So far I was able to get the list view of the external objects with pagination. However when I am trying to access a particular object
I get an HTTP error "400 Bad request". When I inspect the used URL I see that the key property "id" is set to '/x' (%2Fx if encoded) instead of the real id, which can even be seen in the list view.
Here is the decoded URL: https://ecme.empolis.com/oauth/LegoService/LegoService.svc/events?$top=2&$filter=id eq '%2Fx'&$count=true&$select=description,id,sensorId,severity,timestamp,unitId,value
I tried some sample oData4 services provided by odata.org and there the key property is set to its actial value in the same situation.
What can be the reason? How can I fix it?