Here you have a few pointers in the documentation:
https://docs.nextcloud.com/server/stable/admin_manual/issues/general_troubleshooting.html#id1
On top of that, the webserver logfiles can be interesting as well.
If the upload didn’t finish before you download and delete it, this can be problematic. In theory, this should not happen. For the uploads in the documentation, it can problematic if the temp folder is too small etc.