OK - NOW my site works using the EVENTS and NEWS tabs as they should work.
Problems still, in that I cannot get any OTHER posts to appear - but this does not matter to me, since this site is fairly static (except for NEWS and EVENTS).
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Solution Stage 1:
DOWNGRADE Wordpress installation to version 7.2.1, using the instructions at: http://techie-buzz.com/wordpress/downgrade-uninstall-wordpress-28.html
This will allow you to configure the NEWS category correctly in the Admin page. The EVENTS drop-down is still blank.
DO NOT BE ALARMED when you have completed your downgrade, if Wordpress tells you you have to UPGRADE your admin pages when you log in to Admin. This is ok - it is only the Admin zone that is upgraded again, and the back-end is still 7.2.1 (You will see this at the top of the admin screen where you are continuously prompted to upgrade to 8.2.1. Learn to live with it!
The zipped backup of my database needed to be UNZIPPED before it was successfully re-uploaded and imported.
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Solution Stage 2:
You need to find the term_id for the Events category. You cannot set it using the Admin tools, because it is not working, so you have to do it manually.
Your EVENTS category has a "term_id" which you will need. It is an integer number probably between 1 and however many categories you have made (ever). You can use trial and error here - OR go into the database using PhpMyAdmin and look in the table wp_term_taxonomy which contains this field.
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Solution Stage 3:
Get the EVENTS tag working by editing the file function.php (THEME FUNCTIONS) under APPEARANCE > EDITOR. Scroll down to line 185 (although the line numbers in 2.7.1 have now disappeared!!!!) so you can search )(find) the comment line... //Get events
Beneath it you will find the code for the function get_upcoming_events().
It is the line beginning with "$query" that you need to edit. It contains the expression:
WHERE 1 =1
AND wp_term_taxonomy.taxonomy = 'category'
AND wp_term_taxonomy.term_id
IN (
$sero_events_id
)
My term_id for Events category was 10, so I changed the expression above to:
WHERE 1 =1
AND wp_term_taxonomy.taxonomy = 'category'
AND wp_term_taxonomy.term_id
= 10
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My News and Events tabs now both work as expected.
Hope it helps.
If you have difficulties with any of this, let me know if you need more detail.
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I can walk you through the database bits if you need help but probably need to fix a time for real-time chat for this. I used PhpMyAdmin, through the C Panel interface of my hosting company. I had not don it before - although I know databases quite well. "But screw (y)our courage to the sticking place and (you)'ll not fail!) (Macbeth)(-ish)