What is a non$-$waivable conflict of interest?

Mazars has had this client for over $10$ years, why suddenly drop this client, and recommend that no one should rely on the financial statements and tax returns they prepared for this client for the last $10$ years?

Is the concept of $$independence$$ related to a conflict of interest? Has Mazars followed the required code of professional conduct for accountants in the past? Are they self$-$correcting now?

Is Mazars doing the $$right$$ thing by resigning? Is there any similarity in this case in relation to Arthur Anderson and Enron?