Say electron A is nearby another electron (B), so that they may repel each other. Electron B is in a position eigenstate (so it has a definite position). But electron A is not. How does electron A affect the acceleration of electron B? Does it "divide up" its electromagnetic force as if it were a charged object spanning the space that the wave function occupies, whose charge density is proportional to the value of the probability density function? Otherwise, how can electron B decide where to move?
Simply: if an electron can be in "multiple places at once", and the force it produces depends on its location, which location is "chosen" for that force?
...I know that $\exists$ a whole theory on this, Quantum Electrodynamics (thanks Feynman!!!), but I have not studied it. I have only ever taken an intro QM class as an undergraduate.
Edit: If the position eigenstate causes problems, let B be in an arbitrary eigenstate as well. The question is rephrased: if the positions are indeterminate, how is the force, which depends on them, calculated?