Having upgraded my hdd to ssd, I've reinstalled my Electrum watch-only version (Installer old version 3.0.2 that I used to have) and restored it using my MPK.
Are you using the version 3.0.2?
Versions older than 3.3 don't synchronize anymore. Please download the latest version on ELECTRUM.ORG (the ONLY legit website to download Electrum). Anything else is a scam.
I also recommend that you verify your Electrum file signature before doing anything:
https://bitcoinelectrum.com/how-to-verify-your-electrum-download/Yes, I started the same .exe file I downloaded years ago.
It's ok, many thanks for the information and advices.