Hello, @henrik,
That is because when you update the plugin and face this issue, some of the core files of the plugin got missed and that might be because of any server restrictions or incompatibility, so that is the reason you have to upload it manually and it worked for you.
You have to check your server values under XStore >> Dashboard >> Server Requirements, and make sure to set all of them properly and tick green.
And you have to remove the server cache and site cache as well and disable all the third-party plugins including the cache and server cache, when trying to update the plugin, might be any third-party plugin conflicts and removed those files from the Core plugin.
Regards 8Themes Team.