I was having a problem with the Android client not updating files. It would say that it was fetching the latest version of the file, then an Android notification said that the download had failed and the app showed an older version of the file. I’ve recently updated from OwnCloud 8.2 all the way to Nextcloud 13, with a server migration at Nextcloud 12. I can’t say whether the trouble started at any particular transition in there, or if it started after an update to the Android app. Today, it was still happening on NC 13 and client 3.0.0.1. I tried removing and re-adding the account within the client, but it still showed me the same old version of the same file.
After searching here and seeing this topic: App doesn't stay logged in since Nextcloud 3; I tried uninstalling, reinstalling, and logging back in. It worked.
Since the existing topic doesn’t match my exact problem, I figured I’d create a new one that might be easier for someone with my symptoms to find :).