You’ll want to get the full stack trace to find what ultimately triggers the error. It’s tripped in this plugin, but the ultimate cause of that error is always the caller: it’s something that has tried to set up a filesystem connection, but carried on after ignoring an error code, resulting in an eventual fatal error.
i.e. It tries to use a filesystem link without checking first to see if it’s available. That results in questions that can’t be solved (e.g. “does this file exist?” – you can’t answer either “yes” or “no” if the connection wasn’t made, you can only crash instead).
So, the issue has to be fixed in the plugin doing that, as the ultimate crash there is actually the correct thing to do programmatically when asked an impossible question that the caller should not have asked (it should instead have checked the connection first).
Having re-read your description, not all of that is necessarily relevant.
The WordPress updater doesn’t actually call code inside a plugin when updating it. It just unzips the plugin. So there’s probably not an issue inside either plugin – just that your SFTP connection didn’t succeed (and it’s WP that has the issue of carrying on when it shouldn’t, but, that doesn’t ultimately make a difference, since if the connection isn’t there, it can’t update anyway).
You can also update plugins by unzipping them on your computer and uploading them to wp-content/plugins. If you keep seeing the error, then you will want to speak to the person running the SFTP server as to why connections are failing.
