As a management team (consultant), if a contract is in cost plus contract who should pay the rental of scaffolding as needed in the project, the owner or the general contractor? and why?
Contracts can be written to say anything the parties can both legally agree to. Designating a contract "cost-plus", by itself, doesn't say anything about who supplies and pays for structural rebar, soil removal, sanitary reclaim, or, indeed, scaffold rental.
To determine who does what in a contract you have to look at the scope, not its type. The scope should be as detailed as possible. It should spell out who buys from and pays subcontractors and suppliers. The scaffold guys are one of those, and the contract should clearly state how they are hired and paid.
"Cost plus" means, usually, that the contractor gets paid the cost he spends plus an additional fee. The additional fee may be a percentage of total costs, or a fixed amount, or anything else that, again, the parties dream of and agree to. Some contracts have specific provisions for owner-supplied items. These would then be bought and paid for by the owner. The contractor may or may not earn a fee on those, again depending on what the parties agreed to.
So if you are still writing your contract, make sure you spell out who hires the scaffold guys, who pays them, and whether a fee is due to the contractor for that work. If the contract is already written, then read the scope to find out who does what.