cool! I only have the one Manjaro install, my laptop, but I also have another ‘spare’ laptop which currently has endeavour on, and an Ubuntu machine that sits in the corner and serves network storage, printer, etc.
I’m not sure I can help very much on this, but did have a quick google - did you spot this post (below) on the Arch forums? It is old and probably won’t point you straight to an answer, but there are some commands and names of logs, daemons, etc., that you could explore.
https://bbs.archlinux.org/viewtopic.php?id=254251
I suspect that posting more detail on any log information on both the computer and the Nextcloud server may make it much more likely that someone more expert than me may be able to help you or be willing to pitch in.
There’s this really helpful post which often gets suggested to people asking questions on the forum: