download, not install
I had an older version on my external drive and wanted to update it !

The easiest way would be to import that instalation forlder to Galaxy and to updathe through application (or verify / repair).

well the easiest way would be to fix the downloadable version on GOG ! ^^

Well, I have downloaded full offline installer and updated the game. Maybe you don't have enough free space?

I have lot of space and other games offline installers download well

Try this page:
https://support.gog.com/hc/fr?product=gog

On red banner there is information about "user and account refresh". It should help. If not then contact support:
https://support.gog.com/hc/fr/articles/212641829-Je-n-arrive-pas-%C3%A0-t%C3%A9l%C3%A9charger-mon-jeu-que-puis-je-faire-?product=gog

Downloading the offline installer through Galaxy is working fine (confirmed that on my side), so it could be something on your side - ISP problem, firewall, AV, problem with disk drive (also could be a temporary problem installer availability). All of these could lead to such error. Try downloading through web page and contact support.