From a 2008 post by Father Guillaume de Tanoüarn, IBP, on Yves Chiron's book "Frère Roger" [tip: Fecit-Forum]:
Roger loved the Church of Pius XII, even if he took public distance from it at the moment of the promulgation of the dogma of the Assumption of the Virgin Mary, leaving it to his colleague Max Thurian to speak, in good Protestant [language], of "Papalism" in this regard.
Then came the election of John XXIII. They were both received [by the Pope]. The new pope has all [that is needed] to attract them. And, nevertheless, the surprise: Brother Roger would tell Father [Yves] Congar: this pope is not far from being "quite formally heretical". What does he reproach him for? For having declared, during their meeting, that the Church did not possede the entire truth, and that it was necessary to "search together". Curious episode. That was in 1959.
A passage in Roberto de Mattei's "The Second Vatican Council: a history never before written" (Turin, Lindau, 2010):
Among the Protestant observers, there were two brothers from Taizé, Roger Schutz and Max Thurian. When these, during a dinner, asked Hans Küng what they should do in that historic moment [the first session of the Council], the Tübingen theologian answered them: "it is quite well to remain Protestant". A response, [Küng] recalls, that did not please them completely. [p. 227; n. 156]