I believe that this derives, via a game of whispers, from a line in The Idiot, where MuishkinIppolit Terentyev says:
Ailleurs il [Muichkine] dira:† «Je crains de n’être pas digne de ma souffrance.» Et cent autres semblables.
In another place he [Muishkin] says: “I am afraid I am not accounted worthy of my sufferings.” There are hundreds of other sayings all in the same strain.
Eugène-Melchior de Vogüé (1886). Le roman russe, p. 259. Paris: Plon. Translated by H. A. Sawyer (1913). The Russian Novel, p. 253. London: Chapman and Hall.
† Muishkin’sDe Vogüé does not name wasthe character, and the last name mentioned by de Vogüéwas Myshkin on page 258, so if Brandes had been relying on Le roman russe, it would have been tricky to figure out who was speaking in the quoted line, and so he might have written “Eine Person” to avoid getting it wrong.
So, in Russian, Muishkin’sthe line had the temporizing phrase, “а за то” (but for that). In de Vogüé’s translation, this became “je crains” (I fear), a conventional piece of politeness, not an actual expression of fear. In Brandes this became “Ich fürchte nur” (I just fear), the phrasing of which was ambiguous between conventional politeness and literal fear. And in Frankl this became “Ich fürchte nur eines” (I fear only one thing), resolving the ambiguity in the wrong, but more dramatic and interesting, direction.