I know my .htaccess looks a bit odd. I had a long conversation about this with my host. For some reason the only way to avoid internal server error was to put those “RewriteEngine On” and “RewriteBase /site/” lines twice in the file, first to the first lines and then again when the wordpress rules begin. If it isn’t in both places, we’ll have an internal server error… Don’t know why.
Anyway, problem resolved. Thank you!