Getting error #13 on my phone when it tries to sync

For the last couple of weeks (I’ve been too busy to look into it), every time “Nextcloud SMS” tries to sync, I get a notification saying “Nextcloud SMS: Fatal error!” “Error #13: Can’t connect to nextcloud instance” (my translation of what it says in danish, it should be clear what it’s about. Normal access to my nextcloud and calendar+contacts sync works fine from the phone (and also from the computers I regularly use my nextcloud from).

Looking at nginx’s log file (on the server I run nextcloud on) I found lines containing:
“POST /index.php/apps/ocsms/get/apiversion?format=json HTTP/1.1” 405 5 “-” “nextcloud-phonesync (1.3.0)”
(the lines also contain ip, username and timestamp).

Can anyone suggest what to do?

“Nextcloud SMS” runs on a OnePlus5 running OxygenOS 4.5.14 (the newest OnePlus has released) which is based on android 7.1.1.

With the update to the app, I found in f-droid today, it seems syncronisation of sms’es work again.