SUMMARY

STEPS TO REPRODUCE
1. Yesterday I freshly installed Endeavour (with KDE) and wanted to connect my Google Drives
2. I installed KIO as officially recommended by Endeavour, and seting up the accountts worked smoothly (KIO was allowed as an app within my Google accounts, they were showing up in Dolphin etc.)
3. BUT when I tried to access (open) the Google drive folders, an "access denied" error came up and no drive content loaded (while no further warning from Google Drive came up).

The problem seems known for long - when have you planned to fix it? Do we really need to do workarounds for such a basic (core) program?

Here some online discussions around the problem (top results at Google for the described problem):

https://discuss.kde.org/t/kde-online-accounts-not-signing-in/3411/38

You seem to have applied this solution here already (but obviously it's not solving the problem):

https://discuss.kde.org/t/google-blocks-access-from-dolphin-kio-gdrive/26007

I really like Endeavour after first trying it in VM and now having made the first install, but without a good access to Google Drive (where all of my private and professional files reside) it's hard to work with it (Google Drive via Browser is complicated in everyday work, logically).

Thanks and looking forward to any reply!