Time Machine just sticks on 'Preparing backup'

I've seen this problem many times over the years.

Time Machine just sticks on Preparing backup. For hours and hours. Eventually I give up and cancel it.

I see lots of possible solutions on the web but most seem not to work!

Does anyone know how I can check (and understand!) any log file which might tell me what is going wrong? Knowing that, perhaps, will help me find a solution.