If mailbox.getCallbackTokenAsync calls against your Exchange on-premises server suddenly start returning “An internal error has occurred”, you should check if your certificate has expired or was not renewed properly.

Renew certificate

Renewing through your certificate management flow or using the following steps may help you to resolve this issue.

1. Create a new self-signed certificate using the New-ExchangeCertificate cmdlet.
2. Modify the authorization configuration using the Set-AuthConfig cmdlet to set the thumbprint.
3. Publish the certificate created in the step 1.