how about network?

Are you facing any issue related to tempdb?

You can use check constrain.

When SORT_IN_TEMPDB option is ON SQL Server uses tempdb to store the intermediate sort results which are used to build the index while reindexing.

It is always good to have TEMPDB...

I don't think so.

I do not see any blocking.

Could be the permission issue. Any error in SQL log?

What is the status of SQL Server services when you receive this error message?

Disable the antivirus and try.

you can change the query as per your requirement.

can you post result of following command? Where dbname is the database name on which you are facing issue.

sp_helpdb <dbname>

This is normal.

This is due to internal SQL Server processing. Sometime when query plan is available for your query it will executed immediately. Also in the destination table if...

You haven't updated error from sql error log?

Can you restore with command line?

BACKUP DATABASE <DBNAME> TO DISK = '<PATH>'

This is normal and happen when your backpu/copy from primary to secondary will take more than 5 min. So restore job on secondary will not find any file to restore...

Have you checked the error log? What information captured in it? Is this production server? Have you executed DBCC CHECKDB?