Hi everyone,
I am hoping someone from the community can help with this major blocker we have on a Canvas App. I'll try and provide plenty of detail but if anything unclear please let me know.
We have a Canvas App which, OnSelect of an icon on the final screen, should trigger a Flow which sends an email.
The Canvas App has two connections; Office 365 Users and Microsoft Outlook. The Office 365 Connection is working fine as the App is aware of the logged in user and provides visual confirmation of their name and UPN.
The Canvas App is part of our ALM process and goes through Dev, Test and Prod environments. The Flow Owner in the Dev environment is myself; when the Solution goes through Test and Prod the Flow Owner becomes a service account owned centrally.
The issue is:
1. As App author, I am able to fire the Flow in Dev, Test and Prod.
2. Our ALM manager is also able to fire the Flow in Dev, Test and Prod.
3. No other user can fire the Flow in Test or Prod. Instead, they see the following error:

I have tried all the classic fixes:
1. Remove and Re-Add the flow in the App.
2. Ensure the user has selected 'Allow' on the first run of the App to ensure the connections are allowed.
3. I have checked the user group has "Basic User" applied to it.
4. The Flow should be leveraging the user's mailbox to send the email. Therefore the issue should not be related to permissions on a Shared Mailbox.
I have seen posts like this:
Solved: connection not configured for this service - Power Platform Community (microsoft.com)
The problem is, 'Basic User' does not seem to be configurable. When I navigate to it I see the following message:

I think a key point is that when the error is seen in the App, the Flow does not fire at all - IE there is no failed run of the Flow. Therefore the issue appears to be between the App and the Flow.
Hopefully that summarises the main points. Any help on this would be very much appreciated, thank you!