So based on the error message I see it seems that in some circumstances you don't have the rights to send an email on behalf of other email account. Most probably you have an Outlook - Send an Email V2 action where you try to send emails to someone else.
My first thought is to remove the send on behalf for test to see if the problem persist. If the problem is not appear anymore than you can take in consideration to create a new connection in PowerPlatform using the email account from where you want to send mails.
Also, you can re-check the rights you or the connections owner has on the email account for where you try to send the email and make sure there is no one else who can change this rights.
Hope we'll find the solution together ! 🙂
