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The "RPC server unavailable" error can stem from a variety of causes, including network configuration, firewall settings, or RPC service issues.
Ensure the RPC proxy component on the Exchange server is properly configured. You can verify this in the Exchange Management Shell with the following commands:
Get-OutlookAnywhere
Get-ClientAccessServer | fl
Ensure the "Microsoft Exchange RPC Client Access" service is running on the Exchange 2013 server. - You can restart the service to see if it resolves the issue:
Restart-Service MSExchangeRPC
RPC over HTTP, which was not supported by WAF 2.2.0.0-12waf and earlier. So, you can check this also.
More details you can refer to:https://www.sonicwall.com/support/knowledge-base/waf-common-configurations-for-securing-owa-activesync-and-outlook-anywhere-to-access-exchange-mailbox/181218235648455/
Please feel free to contact me for any updates. And if this helps, don't forget to mark it as an answer.