Мне вышеприведённое не помогло. Помогло другое, может пригодится кому.

У меня Windows Server 2019 Standard English, MS SQL 2019 English. Сам сервер и базы имеют кодировку Cyrillic_General_CI_AS.
В Windows был выставлен региональный формат Russian. Под ним и были созданы планы обслуживания, которые не работали.

Полный текст ошибки был такой:

авторDate 2/4/2021 10:23:09 AM
Log Job History (BackUp.Subplan_1)

Step ID 1
Server XXX
Job Name BackUp.Subplan_1
Step Name Subplan_1
Duration 00:00:03
Sql Severity 0
Sql Message ID 0
Operator Emailed
Operator Net sent
Operator Paged
Retries Attempted 0

Message
Executed as user: NT Service\SQLSERVERAGENT. Microsoft (R) SQL Server Execute Package Utility Version 15.0.4073.23 for 64-bit Copyright (C) 2019 Microsoft. All rights reserved. Started: 10:23:09 Error: 2021-02-04 10:23:09.85 Code: 0xC002F210 Source: {83BA8121-DDCA-45C7-87BE-8B76ACABEEE7} Execute SQL Task Description: Executing the query "DECLARE @Guid UNIQUEIDENTIFIER EXECUTE msdb..sp..." failed with the following error: "Culture is not supported. Parameter name: culture 3072 (0x0c00) is an invalid culture identifier.". Possible failure reasons: Problems with the query, "ResultSet" property not set correctly, parameters not set correctly, or connection not established correctly. End Error Warning: 2021-02-04 10:23:09.85 Code: 0x80019002 Source: OnPreExecute Description: SSIS Warning Code DTS_W_MAXIMUMERRORCOUNTREACHED. The Execution method succeeded, but the number of errors raised (1) reached the maximum allowed (1); resulting in failure. This occurs when the number of errors reaches the number specified in MaximumErrorCount. Change the MaximumErrorCount or fix the errors. End Warning Error: 2021-02-04 10:23:10.21 Code: 0xC0024104 Source: Back Up Database (Full) Description: The Execute method on the task returned error code 0x80131501 (An exception occurred while executing a Transact-SQL statement or batch.). The Execute method must succeed, and indicate the result using an "out" parameter. End Error Error: 2021-02-04 10:23:10.24 Code: 0xC0024104 Source: {6B4B944C-50E8-4055-9C21-1106C0765E7A} Description: The Execute method on the task returned error code 0x80131501 (An exception occurred while executing a Transact-SQL statement or batch.). The Execute method must succeed, and indicate the result using an "out" parameter. End Error Warning: 2021-02-04 10:23:10.24 Code: 0x80019002 Source: OnPostExecute Description: SSIS Warning Code DTS_W_MAXIMUMERRORCOUNTREACHED. The Execution method succeeded, but the number of errors raised (2) reached the maximum allowed (1); resulting in failure. This occurs when the number of errors reaches the number specified in MaximumErrorCount. Change the MaximumErrorCount or fix the errors. End Warning DTExec: The package execution returned DTSER_FAILURE (1). Started: 10:23:09 Finished: 10:23:10 Elapsed: 0.625 seconds. The package execution failed. The step failed.

Поменял региональный формат на English (United States). Перезашёл в учётную запись Windows. Создал новый план обслуживания - он работает . Старый так и не заработал, удалил его.

В дальнейшем региональный формат можно поставить снова Russia или какой нравится. Главное, как я понял, под каким создаётся план обслуживания.