Can you see if you have any severe entries in your WfUserWorkQueue or WfPropagationQueue queues? And if so, if you look at the details of any of the entries, do you see anything in the error with the message "ROLLBACK" or anything regarding that a nested transaction cannot occur?
I assume this is also with standard OOTB portions of the workflow, is this correct? Can you also confirm that the objects attempting to be promoted at the time the workflow was kicked off is not either under some other active promotion request or currently checked out?
If you are able to pull some MethodServer log (or BackgroundMethodServer if that is what you have running the queues) text at the time the Conditional task was fired, that would help analyze the error.