My burr grinder suddenly started grinding much coarser at the same setting

Start with the simple things, in this order, because the expensive explanation is the least likely.

Buildup between the burrs. Coffee oils and fines accumulate, and a compacted layer on the burr surfaces effectively increases the gap. This is the most common cause of a gradual or sudden coarsening, and it is free to fix: take the top burr out and clean both burrs thoroughly with a stiff brush. Not a damp cloth — dry brushing, then blow the chamber out.

A burr not seated correctly. If the grinder has been apart, the removable burr must be fully seated in its carrier. A burr sitting on a fragment of coffee or not clicked home sits proud, which coarsens everything.

Something in the chamber. A stone or a foreign object from the beans, jammed between burr and housing.

On these grinders the numbered scale is a relative reference rather than an absolute measurement, so "the same number" does not guarantee the same gap after any disturbance.

One variable people forget when a brew suddenly changes: the coffee itself.

A new bag, a different roast level or a noticeably fresher batch all change how the beans fracture and how fast the bed drains. Darker roasts are more brittle and produce more fines at the same setting; fresher coffee degasses more and can slow a pour-over dramatically.

So before dismantling anything, check whether the bag changed at the same time. It is not the answer here often, and it costs nothing to rule out.

If cleaning does not fix it, the next candidates:

Calibration has shifted. Most grinders of this type have an adjustable zero point — a way to reposition the burr carrier relative to the scale. If the mechanism has moved, the whole range shifts, which matches your description of running out of adjustment at one end. Recalibrating is a documented procedure for most models and takes ten minutes.

Burr wear. Real, and slower than people think — burrs last a long time in home use, typically measured in hundreds of kilograms of coffee. Worn burrs produce a broader spread of particle sizes rather than a clean shift in one direction, so the symptom is usually muddy extraction rather than a sudden change. Given that yours changed over a few days, and that the machine is second-hand, worn burrs are possible but not the first suspect.

A loose adjustment collar. Some designs can drift under vibration if a retaining screw has loosened.

Since you bought it used, there is a diagnostic worth doing before anything else: check whether the previous owner had it apart.

Signs are a missing or mismatched screw, burrs that do not sit flush, or a calibration that is obviously off the factory position. A grinder that has been disassembled and reassembled slightly wrong will work acceptably for weeks and then shift when something settles — which fits your timeline exactly.

Also worth weighing your dose and timing your brew before and after any change, and writing it down. A coarser grind and a faster drawdown are easy to confirm objectively, and it tells you whether a fix worked without relying on taste memory.