I fixed the problem as follows:
I upgraded from jre7 to jre8 (you're probably already correctly on jre8), got the same error message in the original question here except with sqlserver, tried reloading drivers, still got the same message. Then I looked in /home/splunk/etc/apps/splunk_app_db_connect/bin/lib and the only driver was rpcserver-all.jar. I downloaded sqljdbc42.jar from Microsoft (https://www.microsoft.com/en-us/download/details.aspx?id=11774, again, this was for my case using sqlserver), and then reloaded drivers, and restarted the RPC server again, and that fixed the error! (I'm getting a separate error now, but fixed the error message that this question refers to)
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