The server is unavailable and could not be accessed. For a single Web Application / Site Collection

We have recently created 2 new Web Applications with single root site collections. The Web Application name is as follows:

http://myserver:1003

http://myserver:1004

On top of that, we have several other Web Applications with their own site collections.

We have currently experienced an error with the Search Crawl, particularly for the http://myserver:1003. The error is "The server site.com is unavailable and could not be accessed. The server is probably disconnected from the network."

For some reason, this only occurs on the 1003. It successfully crawled the 1004 Web Application.

We have checked and we see that there is already a DisableLoopBackCheck in the registry.

Has anyone a suggestion how to fix this?

Thank you!