Yes, jblanche helped me. The solution was:
metaflac --dont-use-padding --remove --block-type=PADDING *.flac
Because before it looked like:
METADATA block #3
type: 1 (PADDING)
is last: true
length: 423601
after doing
metaflac --list
for each file.
Now every song seems to be playable, but I’ll check my database during the following times.
Thanks everybody who was involved!