In this article you will read what you can do to solve the 500: Internal Server Error.
Microsoft has introduced throttling policies per user. This means that when Microsoft detects that too much data is being downloaded by one user, throttling will take place. In this case the number of connections/downloads will be limited which causes the job to run longer and possibly result in the error mentioned above.
More information about throttling can be found here.
https://docs.microsoft.com/en-us/exchange/client-developer/exchange-web-services/ews-throttling-in-exchange
The workaround recommended by Veeam is to use multiple service accounts. The the backup will be ran with multiple accounts, which should result in less throttling and therefore shorter job completion times.
If you receive the error in the logs, we can help by adding extra users in the backup configuration. First, you will have to create the extra serviceaccounts yourself and add them with the existing account in an Azure security group. If you have created the service accounts, then you can create a case in order for us to assist you. Some further details may be required.