Moving WP from subdomain to its own domain on a new host – WP install broken

Hi,
I am moving a WP install from a subdomain on a host, to its own domain at another host. I followed https://codex.www.ads-software.com/Moving_WordPress, but somehow I can’t get it fixed, I get a blank page.

Details :
Old install @ Host 1 : website is at http[://]websitename[.]olddomain[.]com and WP is in a subfolder (the subdomain is set to get things in that subfolder)
New install @ Host 2 : website will be at http[://]www[.]newdomain[.]com in the root folder

I edited wp_config.php with the database details at the new host, I modified siteurl and home in the database tables (I tried with the old database modified and a database coming from the old site with url options modified), but can’t get it to work. I doubt it is a database problem, but I may be wrong.
I didn’t edit the upload_path field, it was/is empty.

.htaccess is the same at both install :

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

Other information :
Domain name has propagated and point to the right location, I can check if I replace index.php by a customised index.html file that shows up. I’ve tried to use the Search and Replace script to make sure that now old url were left, but that didn’t work either.

I could be missing out sth very simple, but where to start the diagnose ?
Could it come from the fact that the old host didn’t care about adding www whereas the new one does ?
Many thanks for help