Why isn't my App secret working?

Question

I'm creating an app in Confluence, which connects to my Outlook account via the Graph API. I've followed the instructions to register an app on Azure, and added a secret.

The Confluence app was created by my colleague, so he tested it locally with his Outlook, and it worked. Then he passed it to me to install on the main company site. At first it worked fine, but then I made some changes to the Confluence code, and now I get the error:

could not retrieve access token from the provider 401 ({"error":"invalid_client","error_description":"AADSTS7000215: Invalid client secret provided....

I've checked the secret value and it seems correct (and I'm definitely using the value and not the ID). It's hard to tell for sure because when I'm asked to enter it in the command line, it's hidden - is there a way to make it show?

I tried generating a new secret, and that didn't work either (it appears to be accepted ok on the Confluence end, but when I actually try to sync with Outlook, it gives the error above).

I tried creating a whole new App Registration in Azure, but got the same results.

The same app is running fine locally for my colleague. I don't really know what to try next. Is there a reason it would run for one person and not another? Is there anything that can go wrong in pasting the secret into the terminal? We're completely baffled by it as it did work at one point!