I was just searching for the same thing myself - an armhf or arm64 Ubuntu sync client for my nextcloud server (and funnily enough, also for a Pinebook… small world!)… and I simply installed the owncloud-client from the Ubuntu 16.04 LTS repos, and it seems to be working just fine. No issues attaching to the server or getting it to sync, etc.
running ubuntu 18.04 on pinebook pro and nextcloud-client crashes at setup of credentials. thank you for this thread - finally got nextcloud sync working by downloading the owncloud-client instead!