Export was mp4 1920x1080 at 30fps (300MB) as I wanted it to be good quality!
I'm using VP8.40 but it has happened MANY time before on other exported videos using previous VP versions (but always the (near) latest editions). It's a shame I cannot post a photo of the blurry part for others to see.
I'll try the revised export settings and let you know ...
Merci for your quick response.