[qkqw]

On a second note I've simplified your script and made it work with Nextcloud/Owncloud. It makes use of KoboCloud's curl and certificate bundle. I am not quite sure how to check for a connection, but wanted to use NickelMenu like this:

menu_item :main    :Sync Screenshots    :nickel_wifi     :autoconnect
  chain_success                         :cmd_spawn       :quiet:/mnt/onboard/.adds/sync/sync.sh
  chain_success                         :dbg_toast       :Syncing Screenshots

#!/bin/sh

SCREENSHOTS="/mnt/onboard"
KC_HOME="/usr/local/kobocloud"
CURL="$KC_HOME/curl --cacert $KC_HOME/ca-bundle.crt --fail --silent"
HEADER="X-Requested-With: XMLHttpRequest"
URL="[Your Nextcloud/Owncloud instance]/public.php/webdav"
USER="[Your Share]:"

for i in $(find $SCREENSHOTS -maxdepth 1 -type f -name "screen_*.png"); do
  DATE=$(date -r "$i" "+%Y-%m-%d-%H-%M-%S")
  $CURL -u "$USER" -H "$HEADER" -T "$i" "$URL/kobo-$DATE.png"
  if [ $? -eq 0 ]; then
    rm $i
  fi
done

Feedback welcome!