Hello, I have the same problem and I resolved it.

<ORACLE>$ export ORACLE_SID= <INSTANCE>
<ORACLE>$ sqlplus /nolog
SQL> connect /as sysdba
SQL> shutdown abort
SQL> exit

2 processes are not stoped and I kill them.
For me, these processes are: "ora_dbw0_xxx" and "ora_pmon_xxx"

and after-, I can start my database without problem:
<ORACLE>$ sqlplus /nolog
SQL> connect /as sysdba
SQL> startup