Due to a new security implementation added to the 12c database, Toad 11.6 can't connect to the 12c database.
SYMPTOMPS

A user cannot connect to Oracle Database 12c with Toad 11.6. He gets ORA-01031: insufficient privileges when connecting.

CAUSE

Access to SYS.USER$ has been restricted in Oracle 12c

SELECT ANY DICTIONARY Privilege No Longer Accesses Some SYS Data Dictionary Tables

For better security, the SELECT ANY DICTIONARY system privilege no longer permits you to query the SYS schema system tables DEFAULT_PWD$, ENC$, LINK$, USER$, USER_HISTORY$, CDB_LOCAL_ADMINAUTH$, and XS$VERIFIERS. Only user SYS has access to these tables, but user SYS can grant object privileges (such as GRANT SELECT ON USER$ TO sec_admin) to other users.

SOLUTION

The issue is related to the TOAD of v11.6 only. A newer version of TOAD won’t throw the error. To fix the issue, grant access to the SYS.USER$ to the relevant account.

GRANT SELECT ON SYS.USER$ TO "ACCOUNT_NAME";
REFERENCES

ORA-01031 When Connecting To A 12c Database With Toad (Doc ID 1958777.1)

Written At
14 MARCH 201812:40
OEL Release
7.2 x64
Database Release
12.2.0.1.0