Possible reasons for DC fan failure
Here's a couple of ideas that you could investigate fairly quickly...

(1) The fan is an inductive load being switched by a mechanical relay. Could the fan be generating a back emf that damages the fan on switch on or switch off. You can measure this by putting a DSO across the fan wires, triggering on a positive voltage of 26 V and seeing what happens on relay close/open. This is less likely but quick to test for.

(2) The wiring or crimps are faulty and when the motor pulls its stall current at startup, the drop in the supply gives a voltage too low to drive the fan. This is quick to test for using a multimeter across the fan terminals (right at the fan) when switched on.

Hope this helps, please post any test results.