Hello,

For all the time my site will be online I will try to update it with new pages, modules and other configuration. I will try to test these new elements on a local server. But as the site will be online, there will be new data via forum posts, page comments, etc.
For you, what is the best way to synchronise data between the 2 servers?

I have tried WebMatrix but there are some issues when I try to publish the database (because of Yaf Search catalog) and also when I download the online dnn site (WebMatrix wants to create a local user as the one on the remote online server!).
I have also tried to export data with SQL management studio but it didn't work because of contraints.

Since your site has forums and possibly other "user contributed" content, you'll have to take extra care here.

The best think to do is to export modules or pages from the staging server and import them to the production server. Of course, you won't be able to do that for pages or modules that have use content.

For major upgrades, your only real choice is to take the production site offline, make a copy of it, upgrade the copy and use that to replace the production site. Then you can put it back online. This requires planning, rehearsing, and then execution. And, it' best to do that at a time when your traffic is down -- like overnight. And it's always a good idea to advertise the outage in advance.