I’m completely sure about the problem, I tested it in my local server,
Value of RewriteBase depends on 1) htacess location and 2)WP directory. If htaccess is in main directory of WP (default) and WP is located in subdirectory like demotest it should be
RewriteBase /demotest/
So it’s correct, if WP installed in domain.com it should be
RewriteBase /
As I said I’m sure about it, try changing
RewriteRule ^index\.php$ – [L]
to
RewriteRule ^index\.php$ - [L]
If it doesn’t work, provide WP+FTP details as a private reply.