DBMS_SCHEDULER job suddenly running into ORA-24247, except when running manual

I have an Oracle 12.2..1 database on a Windows 10-server that is running a job that is sending 2-5 e-mails a day and has been running for years without any problems.

Now all of a sudden it runs into an ORA-24247 and finally an ORA-600.

When I start the job manually though, it runs fine without any error.

The full error from the alert-file is:

ORA-12012: error on auto execute of job "DB"."JOB_SEND_DAILY_MAILS"

ORA-24247: network access denied by access control list (ACL)

ORA-06510: PL/SQL: unhandled user-defined exception

ORA-06512: at "SYS.DBMS_NETWORK_ACL_ADMIN", line 1454

ORA-06512: at "SYS.DBMS_NETWORK_ACL_ADMIN", line 1454