I have been trying to authorise an addon app (BidSlammer) but hit the following error message-
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Issue with Authorization
The authorization was not successful. If this is your first time to receive this error, please try to authorize again.
If you had an account prior to March 2020, OR you changed your eBay UserID recently, open a support ticket so we can perform a necessary manual step.
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I have had my account for many years so I assume I fall into the "had an account prior to March 2020" category. If so, would you be able to performs the necessary steps to enable 3rd party apps please.

Hello Colin,
Thank you for your question about authorizing. I looked through our log and it doesn't have a session ID. That means something on your end was broken, such as Javascript or cookies.
Could you try again with another browser and let me know? I can check the log again.