The problem seems to be that your SQL Server process has very little free memory left, and the largest contiguous block of free memory is only 983040 bytes. That's why SQL Server is raising the error 'Not enough storage is available to process this command.'.
If possible, try restarting the SQL Server instance to reset the memory allocations, and try running the backup. Then you could run sqbmemory periodically to try and identify which process in SQL Server is taking up all the memory.
If possible, try restarting the SQL Server instance to reset the memory allocations, and try running the backup. Then you could run sqbmemory periodically to try and identify which process in SQL Server is taking up all the memory.