Recently, we encountered a strange problem whereby trying to issue a select over a database link caused an ORA-1089 "immediate shutdown in progress - no operations are permitted" error even though the databases being connected to was up and fully operational.
This turned out to be the result of Bug 13724193, affecting Oracle 184.108.40.206 and fixed in 12.1.
The problem manifests when a database link connects to a primary database that has not been restarted since it transitioned from being an "open read only" standby database to a primary database.
To resolve the issue, restart the primary database (!) that the database link connects to. There is no other workaround documented.