Max coil voltage exceeded
You did not give us a datasheet for the relay, so it's difficult to give a specific answer, but a general answer may be useful. Here's a random 48V-coil Omron relay. 

The coil rating is as follows:


Note the "at 70°C". At a lower ambient temperature, a slightly higher voltage is permissible. That's the temperature in proximity to the relay, not the temperature in the room or outside. Power increases with voltage squared, so even a slight overvoltage results in a much higher power dissipation. For example, 20% over results in a 44% increase in dissipation over nominal. It is possible to allow a bit higher than the 110% if you limit the ambient temperature to less than 70°C. However, that does not allow for changes in the supply voltage that the 110% spec is intended to allow for, so if you get another 10% due to high line voltage, you'll be at more than 50% above nominal dissipation.

The real limitation is the temperature capability of the insulation in the coil, but one must be ever-careful if bending the rules (and even the 110% "spec" is not safe if that's your nominal unregulated voltage). 

If you cannot guarantee that the supply voltage will not sometimes go considerably higher, then I would suggest simply adding a small resistor in series with the relay coil to knock the coil voltage down to something closer to nominal.