Hi En-Tie,
I am glad to hear it works smoothly with “Open with Explorer” mode.
If you have further concerns for troubleshot the issue, I would like to confirm that how did you upload the document when encountering the error? Such as Save as from Office 2010 or Upload files directly on online site?
I tried to upload through different ways on my environment and they seemed working fine. It will be nice if you can provide the detailed steps that you have done and post the screenshot of the error message here for further troubleshooting.
Thanks,
Maggie Li