Snapshot only if source exists/user-callback to interrupt

I'm not sure if I got it right. Are the sshfs mount and the USB stick your sources for snapshots or do you want to store snapshots on them (so they are targets)?

Sorry, I used the wrong word. The network location and USB sticks are my _sources_. My problem is that backintime makes snapshots of the empty mount point if they are not mounted.

You could kill the parent of the parent process with user-callback:

#!/bin/bash
profile_id="$1"
profile_name="$2"
pid=$$

function ppid {
    cat /proc/$1/status | grep "PPid:" | cut -f2
}

case $3 in
    1) #Backup process begins
        if [ true ]; then ###INSERT YOUR CONDITIONS HERE
            kill $(ppid $(ppid $pid))
        fi
        ;;
    2) #Backup process ends
        ;;
    3) #A new snapshot was taken
        ;;
    4) #There was an error
        case $4 in
            1) #ERROR The application is not configured
              ;;
            2) #ERROR A 'take snapshot' process is already running
              ;;
            3) #ERROR Can't find snapshots folder (is it on a removable drive ?)
              ;;
            4) #ERROR A snapshot for 'now' already exist
              ;;
        esac
        ;;
esac

Thank you, this did the trick!

For reference, my script now looks like this:

#!/bin/bash
profile_id="$1"
profile_name="$2"
pid=$$

function ppid {
  grep "PPid:" /proc/$1/status | cut -f2
}
function log {
  logger -t "backintime (user-callback)" $1
}

case $profile_id in
  1) #memory card
    case $3 in
      1) ##backup begins
        if [ ! -d /media/memory-card-mountpoint ]; then
          log "Memory Card not found. No snapshot."
          kill $(ppid $(ppid $pid))
        else
          log "Memory Card found. Making snapshot."
        fi
        ;;
    esac
    ;;
  2) #sshfs remote
    case $3 in
      1) ##backup begins
        if ! ping -c1 -i0.2 -W1 myhost; then
          log "Cannot ping myhost. No snapshot."
          kill $(ppid $(ppid $pid))
        else
          log "myhost pinged. Proceeding ..."
          if ! sshfs -o ro myhost:/mydir /mymountpoint; then
            log "Cannot mount mydir. No snapshot."
            kill $(ppid $(ppid $pid))
          else
            log "mydir mounted. Proceeding ..."
          fi
        fi
      ;;
      2) ##backup ends
        if ! fusermount -u /mymountpoint; then
          log "mydir unmount failed."
          sleep 1
          if ! fusermount -uz /mymountpoint; then
            log "mydir unmount failed, even lazily. Giving up."
          else
            log "mydir unmounted only lazily. Done."
          fi
        else
          log "mydir unmounted. Done."
        fi
        ;;
    esac
    ;;
esac