The editors have encountered an issue while trying to create an A/B test for the start page:
When attempting to create the test, the following error message was received:
"An A/B test has already been created for the selected page."

Here is the message of the REST-API error:
https://<domain_name>/restapi/ab-testing/is-valid?Id=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx&Culture=

400 BadRequest
{
  "ResponseStatus": {
    "ErrorCode": "BadRequest",
    "Message": "An A/B test has already been created for the selected page.",
    "Errors": []
  }
}

However, there is currently no A/B test for this page—neither in Sitefinity nor in Sitefinity Insights.
Additionally, the editors were unable to delete this orphaned A/B test manually.
The datacenter configuration for the site is correct. Also checked for any conflicting A/B tests in the test environment, but none were found. Additionally, the system has been restarted, yet the issue persists.

There is personalization done on the related page. 

The orphaned page data entries in the database that were not properly deleted when the A/B test was removed.

1. Delete the page data that was 'orphaned' because of not properly deleted A/B test. Look in the sf_ab_test_variation table to find the page data entries. The value in 'data_item_id' column is the page_data id. 

2. Delete the page_data entries that have 'abtest' as variation_type_key. First, delete entries from sf_page_data_sf_language_data, then delete from sf_page_data. For example:

DELETE FROM [dbo].sf_page_data_sf_language_data WHERE content_id = 'XXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX' 
DELETE FROM [dbo].sf_page_data WHERE content_id = 'XXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX' 

3. Make sure to restart the application after the DB update above.