I receive this message quite often. It is not a problem of connection and it is not a problem of the ecloud, because I can login to my ecloud account with the browser and download the file that I need.
When I tap on the notification to get a more detailed error message, or at least to read the complete sentence after “Could …” the notification disappear.
I really hope that the developers of the “Files” app start to consider the use case of the people who wants to use the ecloud as storage option without the sync enabled, because the nextcloud android app is really messy, while the Files app works.
Please let me know if I should open a bug on the Nextcloud app github repository