Answer based on context:

The rebellion was ignited by the tax situation, where Eric showed arrogance by not negotiating with the four Estates of the Swedish realm at a Diet. In the summer of 1434, enraged miners and peasants burned the castle of Borganäs near Borlänge. The tension spread, causing several assaults on castles across the country. Nobleman Engelbrekt Engelbrektsson with mining interests in the Bergslagen area stood out as the rebel leader, commanding a peasant army. Negotiations with Eric took place in Vadstena in August 1434, but were unsuccessful. In January 1435 Engelbrekt summoned representatives from the four Estates to a Diet in Arboga, which later has been called the first Riksdag of the Estates. Engelbrekt was elected Captain of the Swedish realm. The antagonism abated when Eric promised changes for the better. However, as before, people felt these promises were not being fulfilled, hence the rebels picked up their axes once more. On April 27, 1436, a rebel army unit was sent marching towards Stockholm, where people still supported Eric due to the strong and influential Danish presence in the city. A certain degree of inner tension among the rebelling forces occurred because the Nobility and Clergy decided to support Karl Knutsson Bonde, who in 1436 had risen to the position of Rikshövitsman. Neither dared remove Engelbrekt completely because of his strong support among the two lower Estates . However, Engelbrekt fell sick and became less active. In a twist of fate highly beneficial to Knutsson, Engelbrekt was assassinated on May 4 by Måns Bengtsson , the cause being an unrelated personal conflict. Consequently, Knutsson won the power struggle . A man named Erik Puke attempted to rally Engelbrekt's old supporters in the Pukefejden, but it was too late. Puke was apprehended and executed in Stockholm in 1437.

Which happened first, the burning of the castle at Borganäs, or unsuccessful negotiations with Eric?
burned the castle of Borganäs