Connection to Oracle Database "as SYSDBA" not working

Good Afternoon,

We have a strange issue where we cannot find a solution.

Our system was migrated to a new sever one month ago (Windows Server 2016 / Oracle 12.2.01). Everything went fine and DB and SAP were running smooth.

But last night, exactly at midnight a problem occured. At 00:00 the last redo log backup could run, at 00:15 the next one failed because it could not login "AS SYSDBA".

We checked this then and saw that this is not working for any user:

• sqlplus / as sysdba --> Not working
• sqlplus system/*pw* --> Working
• sqlplus system/*pw* AS SYSDBA --> Not working

Error is always the same:

ORA-01017: invalid username/password; logon denied

We checked note 480266 but everything is fine:

• <SID>adm is part of ORA_DBA and also ORA_<SID>_DBA
• SQLNET.AUTHENTICATION_SERVICES = (NTS) in sqlnet.ora
• Environment variables are set correctly
• TNS config seems to be correct (worked for the last 4 Weeks, SAP still running smoth)
• <SID>adm is not locked or disabled
• Connection to AD Server seems to be fine

We saw that there is no orapwd File, but as we understand sqlplus / as sysdba should work anyway localy on the server.

Any other idea what the problem could be?

Thans and best Regards