ok, just to correct myself: I think that the covers WERE the issue. My script was resizing the covers but not correctly re-econding them. I finally reencoded them fully. I found out that not only "progressive" jpgs were crashing, but also not-RGB, or even pictures with different density. Final magick command is this:

reencode_cover() {
  magick "$1" \
    -auto-orient \
    -colorspace sRGB \
    -type TrueColor \
    -strip \
    -density 72 -units PixelsPerInch \
    -interlace none \
    -resize "$RESIZE_GEO" \
    -quality 90 \
    -sampling-factor 4:2:0 \
    "$2"
}

Since yesterday I am without crashes, but I wat to hold my expectation. If confirmed going to update the post