The answer stated by @Mulvya (thank you!) works: “Add -safe 0 before -i“.
Then another problem appeared with find STREAM -name '*' -printf "file '$PWD/%p'\n" which returns the empty path as first entry.
I changed this for for f in ./*.wav; do echo "file '$PWD/$f'"; done (see https://trac.ffmpeg.org/wiki/Concatenate) and now it seems to work. Hurray!