This is what I feared. So one way to improve this on the duplicati side would be to treat 423 as a soft error, instead of a hard error. But I don’t have enough knowledge of how things work in duplicati to judge if it is this easy.
Let’s assume for a second that there’s no bug in nextcloud and no bug in duplicati. Question is then if there’s anything which could lock the files. Any other sync clients or something. But this can only be answered by @Adramelramalech. One test could be to use another nextcloud account, just for backups. And see how this plays out.