UPDATE
Ok, so here's what I did this morning:
- Explored OneDrive\Applications\Joplin and found out the folder was 138Mb! Too large for a "failed" sync...
- Copied all files on my laptop for security sake
- Opened Joplin>Settings>Synchronization and set the sync target back to OneDrive
- Selected
Delete local data and re-download from sync target
and BAM! All notebooks are flowing back into my local Joplin instance!
Lessons to learn:
- Always do backup before any change, whether dubbed critical or not
- Always clear data first, so you know what's the initial context
- Keep trying, IT is not an exact science and something could finally work!
- Be humble and warmly thank all you provided help and guidance. So I do.
By the way, I really don't understand what happened yesterday... 
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