Embrace the content type which contains space within double quotes like "News Page Channel".
http://mysite/_api/search/query?querytext='contenttype:"News Page Channel":Corporate'&rowlimit=1
This worked for me. I hope it works for you also.
Note: Any parameter with space in search query should be embraced with double quotes( " ).
Just like willman said, you can have a try with managed property to search. The link he provide is quite useful.
Just add some contents about searching, you can use the KQL(Keyword Query Language) to build your own query for searching. You can make the end users learn it or create custom search web parts with a query to improve user experiences.
If you have Managed Properties setup for the fields you want to search, then you can search by specific field. Be sure to read the section in that linked article titled "Define which content that users can search and get results for".
Once that is configured, the user can search by specific field in the search box, by typing the name of the searchable ...
Quoted from this article:https://docs.microsoft.com/en-us/sharepoint/search/search-schema-overview
Searchable: Enables querying against the content of the managed property. The content of this managed property is included in the full-text index.
Example: If the property is "author", a simple query for "Smith" returns items that contain the word "Smith" and ...
Are you certain you can get it to work by changing "." to "_" or changing to "_DD.MM"?
“_MM.DD” or “_DD.MM” should be identified as the same format by SharePoint. Only ISO 8601-compatible datetime formats are supported in queries, such as “YYYY-MM-DD”.
Per my test, what is returned in the search result is based on what keyword you have used.
Where do the users search from? From search center or from a site search? Please offer a screenshot about the error page.
Try to create a new search center and search from the new search center, compare the results.
Make sure search related service is started.
Make sure your Search service application is connected to web applications.
Navigate to ...
Please check your TCP IP 808 port configuration both in WFE and Application servers. Other Port need to be checked TCP 32843 and 32844
For all required port details in SharePoint 2016, please refer the below article :