If you get the following error:

[Google Contacts]Uploading item(s) to folder 'account@gmail.com/Contacts' failed due to the following error: Execution of request failed: https://www.google.com/m8/feeds/contacts/default/full (Invalid request.)

The first thing you should know is that most of the "folders" of Google's mail, contacts, tasks, calendar, etc. are just a complex Tag system that's pretending to be folders.
When objects (messages, events, contact entries etc.) are without a Tag to sort them somewhere, strange things and errors start to happen. Understanding that will help you understand the rest of the explanation and the solution.

It seems that if you click the orange + New > Contact button then it will typically default to the root /Contacts folder, which is what's causing your error. If you're creating a new contact using that method, you must either select a Tag to apply to the folder or select a subfolder.

The way to change the default is to select any of the subfolders before clicking the orange + New > Contact button. Then it will automatically apply a tag to the contact and it won't trigger the error.

To fix the rest of your contacts that have this "No Tag" error, the easiest thing to do would be to go to the Contacts folder and sort all of your contacts by Tag in ascending order. This will cause all your "unsorted" contacts to appear at the top. Drag all of the unsorted contacts into any of your subfolders and this will apply a tag to them and stop the error. It could also be helpful to click the "select" option so that you can easily select multiple entries at once.