After many trial and error, I have concocted a solution for this...

BEFORE uploading on DTube, ensure the video is in mp4 file and thumbnail size is below 500kb.

After doing so, please follow the step-by-step instruction:

1. Upload your video
2. After your video being uploaded successfully with a GREEN TICK, you may upload your thumbnail
3. After your thumbnail being uploaded successfully with a GREEN TICK, edit your title
4. Later, edit your description
5. At last, tag your video in the following format (all letters must be in smaller case): "dtube[space]error[space]submitting[space]to[space]blockchain"

Viola! I wish you in the best of luck uploading your videos on DTube. Cheers. :)