After upgrading Sitefinity to 15.4 from 15.1, 'sign in' popup keeps coming up when trying to access 'pages' :

The site has recently been upgraded from Sitefinity version 15.1 to 15.4

The issue is caused by orphaned page data records in the database that referenced non-existing page nodes.

1. Run the SQL query to identify the orphaned data records:

SELECT content_id, * FROM sf_page_data WHERE page_node_id NOT IN (select id from sf_page_node)

2. Delete the identified orphaned page data records using the provided API code (see the attachment OrphanedPages.aspx). Place this webform code at the root of the website project and request it by Url <site_name>/orphanedpages.aspx.

3. If the delete action from step 2 fails, proceed with the following steps below (a, b, c, d). These queries will create temp table, copy existing page node record and update its id with a missing one for the page data. Then copy the temp record to the original table sf_page_node and will delete the temp table:
           a. In the backend create one temp page (to ensure will not mix with any existing one). For example create page with title "TestTemp". You can delete it once we fix the issue.

b. get the ID of that temp page from sf_page_node, example:
    

select id, * from sf_page_node where title_en = 'TestTemp' 

c. Get the list of page data IDs as before (save the result for further reference):

select content_id, page_node_id, * from sf_page_data where page_node_id NOT IN (select id from sf_page_node)

d. Run the following queries at once, where change the ID in the first line with the one of the "TestTemp" page from step b above:

SELECT * INTO sf_page_node_tmp FROM sf_page_node WHERE id='GUID_ID_OF_TESTTEMPpagenode' -- ID OF AN EXISTING PAGE NODE TO COPY THE DATA
declare @pn_Id uniqueidentifier
SELECT TOP 1 @pn_id = page_node_id from sf_page_data where page_node_id NOT IN (select id from sf_page_node)
UPDATE sf_page_node_tmp SET id = @pn_Id
INSERT INTO sf_page_node SELECT * FROM sf_page_node_tmp
DROP TABLE sf_page_node_tmp