SYMPTOM
Strategy Narrowcast Subscriptions fail to load, and when checking the DE.log file at the time of the reproduction, the following entries can be seen:
[XXXXXXX::SBM]:4980:[THR:0x918][12/08/2018 03:00:03.598][Level:Info (SBM)][Code:0x00002494]Error Connecting to Database for Pre/Post SQL statement: Before - Invalid authorization specification (User ID or Password is incorrect). [XXXXXXX::DM]:1076:[THR:0x490][12/08/2018 03:00:07.717][Level:Moderate Error][Code:0x00000A98]Service ID [WWWWWWWWWWWWWWWWWWWWW] Execution ID [YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY] Segment ID [........]Error running pre-sql [Subscription Set ID: ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ] [XXXXXXX::DM]:1076:[THR:0x490][12/08/2018 03:00:07.717][Level:Moderate Error][Code:0x00000963]Service ID [WWWWWWWWWWWWWWWWWWWWW] Execution ID [YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY] Segment ID [........]Service execution finished with error.

ACTION
Update the credentials for the DSN(s) and connection information for the pre-execution SQL and post-execution SQL statement definitions in each of the Subscription Sets defined
And if you still encounter the following error, when testing it:

Unable to connect to the database. Please make sure the login, password, and database name are correct.