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The exception simply means that Microsoft Excel is not in a right state where it can be used.
The key is to check the runtime log for the penultimate execution on the Runtime where you are getting the exception. Once that root cause is resolved, the 'RPC Server.....' exception, which is simply occurring as a consequence would go.
Please check the logs. Revert in case you need any help. Would be happy to assist.
Recently, I have something which leads up to this issue.
If an automation fails while executing a remote procedure call (RPC), such as a VBA macro, we get an exception explicitly saying it. Unless we restart runtime, the subsequent automations which use Excel fail with the error as 'RPC server is unavailable'.
Need to find the root cause for why does it need restarting Runtime in order to be cured.