No,there is no problem if the core especially the upper yoke has a temperature of 120°C even on an idle run in Resiglas dry type transformers .It can even have temperature greater than 120˘C. The maximum permissible temperature of the core is defined only by the temperature withstand parameters of other parts of the transformer. Standard temperature protection for windings (class F) is set on 145° for alarm and 155°C for tripping.