My students have regularly been writing sentences which baffle me grammatically. They write a simple or compound sentence, then they follow it with the coordinating conjunction but followed by the subordinating conjunction because. Can because follow but in a sentence? I think it is a jumble of conjuctions, but I am not sure if this is acceptable because it is done so often.
Naturally you wouldn't accept this if you believe you shouldn't start a sentence with 'but'. But otherwise I can't see a problem.
This is similar to "However, because he had ... ".
I would say that, strictly, a comma after 'but' is necessary, though no ambiguity exists, as it does with 'however', however.