When starting up WebLogic after a network outage, we received the following exception:

oracle.security.jps.JpsException: JPS-01050: Opening of wallet based credential store failed. Reason java.io.IOException
        at oracle.security.jps.internal.config.OpssCommonStartup.preStart(OpssCommonStartup.java:423)
        at oracle.security.jps.JpsStartup.preStart(JpsStartup.java:389)
        at oracle.security.jps.wls.JpsBootStrapService.start(JpsBootStrapService.java:80)
Caused by: oracle.security.jps.service.credstore.CredStoreException: JPS-01050: Opening of wallet based credential store failed. Reason java.io.IOException
        at oracle.security.jps.internal.credstore.ssp.CsfWalletManager.openWallet(CsfWalletManager.java:191)
        at oracle.security.jps.internal.credstore.ssp.WalletCredentialStore.doInit(WalletCredentialStore.java:170)
Caused by: java.io.IOException
        at oracle.security.pki.OracleWallet.open(Unknown Source)
        at oracle.security.jps.internal.credstore.ssp.CsfWalletManager.openWallet(CsfWalletManager.java:179)
        ... 31 more
Caused by: java.lang.ExceptionInInitializerError
        at oracle.security.pki.OracleFileSSOWalletImpl.a(Unknown Source)
        ... 33 more
Caused by: java.lang.RuntimeException: java.io.IOException: Read-only file system
        at oracle.security.pki.FileLocker.(Unknown Source)
        ... 34 more
Caused by: java.io.IOException: Read-only file system
        at java.io.UnixFileSystem.createFileExclusively(Native Method

I tried manually opening up the wallet but received a "ewallet.p12 not present" error:

oracle@soahost:/home/oracle> $MW_HOME/oracle_common/bin/orapki wallet display -wallet $DOMAIN_HOME/config/fmwconfig -complete

Oracle PKI Tool : Version
Copyright (c) 2004, 2019, Oracle and/or its affiliates. All rights reserved.

ewallet.p12 not present at /u01/oracle/domains/soa_domain/config/fmwconfig

My first warning sign was when I ran the df command to report file system disk space usage, it actually hung:

oracle@soahost:/home/oracle> df -h

Then the root cause of the issue was finally identified:

oracle@soahost:/home/oracle> touch /tmp/z
touch: cannot touch `/tmp/z': Read-only file system

The /tmp folder was in read-only mode. Oracle WebLogic Server requires read/write to the /tmp folder, as you will notice adhoc product files created there usually.