Are your two schedules set up so that refresh activity between the two jobs might overlap in time? That is, could the incremental and full refreshes actually be happening at the same time? I know this isn't supposed to create a problem (only one of the two should actually happen at one time, regardless of how they are scheduled) but I'm wondering if that logic might have suffered a regression.
Can you try adjusting the start times of the jobs to prevent any possible processing overlap and see if the problem still happens?
Also, you should verify that each job, on its own, has no problem running. If that's true, then we can be pretty confident that it's the interaction between the jobs that's creating the problem.