Major Luther? He was a military chaplain?

All kidding aside, Reuters tells us "Germany Catholics wary about major Luther festivities". 2017 will be the 500th anniversary of the Reformation, and the party is already in the works. Catholics are invited, but for some reason feel a little hesitant.

(RCIA inquirer: "Luther had some . . . interesting ideas." Me: "Well, we're a little biased about him here.")

And this:

Margot Kaessmann, a former Lutheran bishop who heads the preparations for the 2017 events, has said she wants Catholics to join in but turned down a Vatican suggestion both sides work out a common admission of guilt for the separation.

What does she object to, "common" or "guilt"? Or does she just not want to spoil the festive atmosphere?