Discovery did not complete causing the error from the Discovery Log:
"SNMP probe timed out. Target is either unreachable or there are no valid credentials for it." 

This issue happens after an instance clone, or clone then upgrade, if the Clone Preserve/Exclude settings do not include all child tables of discovery_credentials.

Discovery Credentials - Orphaned cause "SEVERE *** ERROR *** An error occurred while decrypting credentials from instance"

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The error message "SNMP probe timed out. Target is either unreachable or there are no valid credentials for it." is only seen in the Discovery log but if the "agent0.log" (MID Server log) is checked, the following exceptions are found:

11/26/18 12:10:22 (830) Worker-Interactive:SNMP-8d6cea70db422300f2a328664a9619c3 DEBUG: Flushing session cache due to credential load.
11/26/18 12:10:22 (939) Worker-Interactive:SNMP-8d6cea70db422300f2a328664a9619c3 SEVERE *** ERROR *** An error occurred while decrypting credentials from instance
com.snc.automation_common.integration.exceptions.AutomationIOException: Unable to retrieve data from instance. This MID may not be validated.
at com.glide.util.MIDServerInfoPayloadDecrypter.decryptPayload(MIDServerInfoPayloadDecrypter.java:25)
at com.service_now.mid.creds.provider.standard.StandardCredentialsProvider.loadCredentials(StandardCredentialsProvider.java:289)
at com.service_now.mid.creds.provider.standard.StandardCredentialsProvider.load(StandardCredentialsProvider.java:256)
at com.service_now.mid.creds.provider.standard.StandardCredentialsProvider.loadIfNecessary(StandardCredentialsProvider.java:268)
at com.service_now.mid.creds.provider.standard.StandardCredentialsProvider.iterator(StandardCredentialsProvider.java:137)
at com.service_now.mid.probe.SNMP.getCredentials(SNMP.java:755)
at com.service_now.mid.probe.SNMP.attemptSNMPv3(SNMP.java:405)
at com.service_now.mid.probe.SNMP.attemptSNMP(SNMP.java:340)
at com.service_now.mid.probe.SNMP.probe(SNMP.java:235)
at com.service_now.mid.probe.AProbe.process(AProbe.java:96)
at com.service_now.mid.queue_worker.AWorker.runWorker(AWorker.java:125)
at com.service_now.mid.queue_worker.AWorkerThread.run(AWorkerThread.java:20)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)

11/26/18 12:10:23 (033) Worker-Interactive:SNMP-8d6cea70db422300f2a328664a9619c3 DEBUG: Flushing session cache due to credential load.
11/26/18 12:10:23 (126) Worker-Interactive:SNMP-8d6cea70db422300f2a328664a9619c3 SEVERE *** ERROR *** An error occurred while decrypting credentials from instance
com.snc.automation_common.integration.exceptions.AutomationIOException: Unable to retrieve data from instance. This MID may not be validated.
at com.glide.util.MIDServerInfoPayloadDecrypter.decryptPayload(MIDServerInfoPayloadDecrypter.java:25)
at com.service_now.mid.creds.provider.standard.StandardCredentialsProvider.loadCredentials(StandardCredentialsProvider.java:289)
at com.service_now.mid.creds.provider.standard.StandardCredentialsProvider.load(StandardCredentialsProvider.java:256)
at com.service_now.mid.creds.provider.standard.StandardCredentialsProvider.loadIfNecessary(StandardCredentialsProvider.java:268)
at com.service_now.mid.creds.provider.standard.StandardCredentialsProvider.iterator(StandardCredentialsProvider.java:137)
at com.service_now.mid.probe.SNMP.getCredentials(SNMP.java:755)
at com.service_now.mid.probe.SNMP.attemptSNMPv1_v2c(SNMP.java:361)
at com.service_now.mid.probe.SNMP.attemptSNMP(SNMP.java:346)
at com.service_now.mid.probe.SNMP.probe(SNMP.java:235)
at com.service_now.mid.probe.AProbe.process(AProbe.java:96)
at com.service_now.mid.queue_worker.AWorker.runWorker(AWorker.java:125)
at com.service_now.mid.queue_worker.AWorkerThread.run(AWorkerThread.java:20)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)

The error is encountered every time the credentials are being loaded in the system and that happens when Discovery runs.