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I can see in the log file that everything works well, and we do receive the expected responses from Google Drive. Therefore, the file has been uploaded. The question is where.

You need to understand that Google Drive was never meant to be generic file storage, let alone file storage that maps to a filesystem. It was made to store Google Docs. The general purpose file storage was grafted onto it haphazardly at a later point in time, when it became apparent Dropbox (and SkyDrive which later became what you now know as Microsoft OneDrive) was eating Google's lunch.

The organisation of Google Drive is not based on folder names. You just have folders known by their internal IDs. Each folder has a parent folder ID and a label. What you see in the interface is the label. Nothing stops you from creating two identically named folders in the same place. Deleted folders simply have a flag set on them. It is, therefore, possible to have two or more identically named folders, all with the same parent folder, one or more of which may be deleted (i.e. in the Trash).

Is it possible that you have a folder identically named to your desired storage folder elsewhere on your Drive, e.g. in Trash, or Shared?

No worries! The very weird way Google Drive works with folders has caught a lot of people out, including yours truly. The first time it happened to me I thought I must be really stupid or far more tired than I thought I was, because after two hours I couldn't see anything wrong with my code. It took another half hour of sleuthing to find out that the problem was that the first ID returned for a specific folder name in Google Drive may not match what their interface shows as the intended-named folder. Yes, when there are duplicate folders with the same name their interface will show them with a suffix like "(1)".

Ah, Google, only marginally saner than Microsoft. Sigh.

Yeah, sanity and Google are strangers to each other. If you haven't seen any folders becoming created afresh into the Trash you're one lucky man.

I recommend staying away from consumer-grade storage like Google Drive, OneDrive, Dropbox etc if you value your sanity. Instead, I recommend using S3-compatible storage. Not necessarily Amazon S3 proper; I am talking about S3 as an API protocol. There are plenty of third party storage providers offering S3-compatible storage including a lot of European ones: Scaleway, IONOS, Hetzner, etc. These all do work perfectly with Akeeba Backup, and they even tend to cost less than the consumer-grade storage providers! If you're worried about synchronizing them with your local computer, fear not for there's software like Mountain Duck (macOS and Windows, made in Switzerland) and rclone (Linux, Windows, macOS – it's CLI, but there's a GUI available as well). 

You're welcome!