The Pi4 is still a small device, and with a few users, or with some synchronous syncs you can easily saturate this device. It really depends on a use case. Also some apps can add an enormous load to it. For a somehow limited use-case it is an inexpensive and energy-efficient Nextcloud server. However wrong disk, no good power supply and bad settings can quickly degrade performance. NextcloudPi and the documentation around help you to avoid this.
And the pi4 is already a few years old, there are other small devices to run Nextcloud: What modern hardware do you recommend for NextcloudPi?
That would be interesting to know. In the logs, there can be hints if the transfer was interrupted or aborted.
Sure, but we need to know if they client just “forgot” to switch the symbol to green or if it never received the transfer confirmation. When you upload a file, you can check the webserver error log:
That would be for the query that uploads the file, e.g.
123.456.789.12 - - [09/May/2023:10:04:12 +0200] nc.example.org "PUT /remote.php/webdav/temp/del052023/TextFile.txt HTTP/2.0" 201 0 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0) Gecko/20100101 Firefox/112.0" "-"
Here you got status code 201 for the PUT request to upload a file.
I’d try to upload more file, so it is failing again and check the webserver logs. Not sure what is happening then, you might be able to adjust the configuration to avoid this. And if the issue is known, report this to the desktop bug tracker that the client might be able to identify such a situation and reduce the parallel uploads.
Or other idea: get test account on try.nextcloud.com, add a sync client and upload a few files if you can reproduce. Then it is easy to report and reproduce the problem since it isn’t linked to your setup.