The correct sentence is the second one: "The concert was due to start at seven o'clock, but the conductor didn't come until half past seven." This means that the concert was scheduled to start at seven o'clock, but in fact it didn't start then. The start of the concert was delayed because of the late arrival of the conductor. The first sentence does not make any real sense. 'About to start' describes a situation at a particular moment, when something was on the point of happening. For example, you might say this: "It was seven o'clock and the concert was about to start. The orchestra members were ready to play and the audience was waiting. Then somebody realised that the conductor had not arrived."

Hi Cathy! 1. The concert was about to start at seven o'clock, but the conductor didn't come until half past seven.

Because 'due to' should only be used as an adjective and After a noun. So in here we can used 'about to'.

The second sentence IS right, Cathy. See my answer below!

Thank you for your help! But why is the second sentence wrong? I still feel confused. According to my understanding if something is due at a particular time , it is expected to happen at that time. There is the usage of "be due to do sth". For example: The committee was due to meet on October 1st. I think in the second sentence "the concert was arranged or expected to" start at seven o'clock is acceptable.