In its issue no. 25/26 of 26 June 1918, the magazine Signale für die musikalische Welt had a news report from Lübeck: “As guest conductor, Furtwängler — to whom the Lübeck public is greatly attached on account of his activity in the past — conducted The Flying Dutchman with rare force of conviction”.
What about this: an unreported opera performance! There is no date, even if it is clear it was during the spring of 1918, but there is no trace in the known records. Nothing in what has been published concerning his activity in Lübeck and nothing either in his correspondence with Lilli Dieckmann. The nearest we find is this reference in a letter he sent her on 2 May: “It was a pleasure seeing you again in Lübeck…”
So, if you have any info…
Along the same lines we have discovered an unreferenced concert in Landau on 29 June 1920, Furtwängler conducting the Symphony Orchestra of the Palatinate.