I found the cause. The “SC ExtMech To Process” custom record type had over 4000 entries. It looks like this may be a defect in the extension manager. It appears it creates several records each time an activation is run. When the edit link is clicked, it appears the extension manager is performing a search of this custom record type and there is a limit of 4000 records. The extension manager needs to remove the very old records. Or modified the search to only retrieve the latest records.

I would recommend raising a case with support so our team can investigate