Can't login - "unexpected error has occurred" ORA-01502: index 'PROTECT.SYS_C00

The error indicates that a particular index mentioned needs to be rebuilt.

Indexes can be rebuilt using the following command run as the user which owns the index:

ALTER INDEX <index_name> REBUILD;

To identify indexes that need to be rebuilt, run the following as SYS:

SELECT INDEX_NAME, STATUS FROM DBA_INDEXES WHERE STATUS != 'VALID' AND lower(Owner) = 'protect';