The explanation is not quite accurate since 15K partitions were supported in SQL Server 2008 SP2 and SQL Server 2008 R2 SP1.

Explanation

The partitioning limit for SQL Server 2019 for tables...

13.0.1600.12

You can find it here: C:\Windows\SysWOW64\SQLServerManager12.msc

The 32bit version of DTExec still gets installed with SQL 2012 and for a default install can be found here:

C:\Program Files (x86)\Microsoft SQL Server\110\DTS\Binn\DTExec.exe

Not sure about the Excel issue.

Distributed transactions for AlwayOn will be supported in SQL 2016 but not SQL 2012.

https://msdn.microsoft.com/en-us/library/ms366279(v=sql.110).aspx

Example: Setting Up Database Mirroring Using Certificates

https://msdn.microsoft.com/en-us/library/ms191140(v=sql.100).aspx

I would start here: http://www.sommarskog.se/query-plan-mysteries.html

As DouglasH mentioned, it could have been a restart of SQL Server Agent or someone may have run sp_cycle_agent_errorlog.

You could setup a SQL Agent job on each replica to run the following:

DECLARE @ServerName NVARCHAR(256) = @@SERVERNAME

DECLARE @RoleDesc NVARCHAR(60)

DECLARE @AGName NVARCHAR(256) = 'YourAGNameHere'

SELECT @RoleDesc = rs.role_desc

FROM...

You could kill the SPID running the job. I modified your query to include the SPID.

SELECT s.session_id,jobs.name AS [Job_Name]

, CONVERT(VARCHAR(23),ja.start_execution_date,121)

AS [Start_execution_date]

, ISNULL(CONVERT(VARCHAR(23),ja.stop_execution_date,121), 'Is Running')

AS [Stop_execution_date]

, CASE ISNULL(CAST(ja.stop_execution_date AS VARCHAR(30)),'NULL')

WHEN 'NULL'

THEN...

You would need to share the folder on the linked server and grant either the SQL Server service account (if running the backup through SSMS) or the SQL Agent service...

I've seen this happen when SQL is installed on a server and then the server is renamed. Even if you run sp_dropserver and sp_addserver, the syspolicy_purge_history job is not updated...

It sounds like the windows firewall is probably blocking the ports for SQL Server. In control panel go to Windows Firewall and open UDP port 1434 for the SQL Browser...

I'm assuming transactional replication since you didn't specify and my guess is that it's waiting for the snapshot to be taken. Look under SQL Agent jobs for a job with...

Seems this is a know issue. Here is the connect item on it:

https://connect.microsoft.com/SQLServer/feedback/details/761308/xp-enumerrorlogs-returns-incorrect-results-when-running-on-windows-server-2008-r2