And when I login using my Sandbox user account, I am being asked to Approve my app, however clicking "Approve does not seem to do anything.
In the background I can see that there is a POST done to: https://settlit1.sandbox.freeagent.com/app_approvals?method=post
However it seems like the page itself just refreshes itself.
When I check the user account on FreeAgent, my app is listed as having access.
I must be doing something wrong, but I’m not sure what. Thanks in advance,
I had a quick look at the handoff URL you provided, and it seems on first glance that the redirect_uri is doubly encoded - decoding it gives the following: