Event not found error only on classes

OK I'm extra puzzled, the new event does not show the error so something has happened to the classes that were in the database but isn't happening on new ones. I'll keep digging but I think there's not much point you putting more effort in until I find something useful. Thanks for the pointers.

why not moving the copied site in the place of the original, if it works there …

The local copy worked with an old version of the database, the latest version of the database displays the error. There is a problem in the data that is triggering the error but only on existing classes not new ones although I can see no difference between an old class record and a new one. The code that generates the error seems to be checking categories and registration but there is no registration. Occurs around line 164 in event.php (models).

Using the joomla debugger I copied and ran the SQL queries that were jem related directly on the database (local copy) and no error occurred. I have given up for the moment as my efforts are getting a bit wild. I have resorted to making the 404 a 403 so it doesn't show. I'll keep having a go every now and then and will report back if I make progress. I've changed the title to reflect where I'm up to.

If I go to the UKJive classes, I don't see the error now (you've hidden it). I don't have more data, but I think the error might be that there is an event with a category that doesn't exist (deleted in the past). For this reason in a new event, you don't have the issue.

Try this.
You can check this script in mysql:

SELECT e.id, e.title
FROM `xxxxx_jem_events` as e
INNER JOIN xxxxx_jem_cats_event_relations as cer ON cer.itemid = e.id
LEFT JOIN xxxxx_jem_categories as c ON c.id=cer.catid
WHERE c.id is null;

Change xxxxx to your Joomla prefix in the database.

If you have any events in a category that doesn't exist, you will see the event list (id, name) with an incorrect category. Edit the event and update the category.