This event indicates that all message blocks are in use and that the session had to wait for a message block to become available.
Related posts
12c New Features – Database In-Memory
Resolving tablespace issues (ORA-01650 / ORA-01652 / ORA-01653 / ORA-01654 and ORA-01658)
Troubleshooting Listener Startup or Connection Hang issues in 11g.
Listener Issues ORA-12505/TNS-12505 or ORA-12514/TNS-12514
Oracle datapump Architecture & Internals
Oracle datapump Internals
Install & Configure Oracle XE on Linux/CentOS in less than a minute.
Resolving ORA-00845: MEMORY_TARGET not supported on this system
How to read Datapump Dumpfile Headers
Oracle Database Internal Mechanism 