I was using strap 3.0.0.next-11 and then migrated my APIs to 3.6.8 version.
In 3.6.8 i see this error in a pop-up , for collections which has relations:
An error occurred during models config fetch.
on logs i see this error :
Cast to ObjectId failed for value "http://54.179.156.135:1339/uploads/d26af51633f2451a934896bfc125ec90.jpg" at path "_id" for model "file"
Why is this happening on 3.6.8 ? I have been using the older version without any issues and with this new version I am unable to feth anything.
I am using following : node : 14.17.6 (LTS) npm : 6.14.15 strapi : 3.6.8
I have also attached the image of my package.json.

So I figured out the reason why this was happening in my case. After migrating to 3.6.8 , the fields in model which have type :
need to have values stored as ObjectId in database as a reference to an entry in upload_file which is maintained by strapi internally.
earlier,
thumbnailwould store value as a string url ( url of the image ).Example :
Now ,
thumbnailstores the reference i.e.ObjectId, which refers to an entry in theupload_filecollection which is responsible for maintaining all the images uploaded via strapi upload api.Example :