Microsoft has just informed that,

We discovered this morning that there was an error in the database for provisioning record data, which is required for new phones to join the Fast Ring through the Windows Insider app. The team has been working to correct it but the load on the provisioning data servers has made all operations extremely slow. To fix the issue, we are taking the service offline and updating the provisioning record data for all 5100 phone POPs. It will take approximately 2 hours for us to make the update, and validate that everything is working. This is a different service from Windows Update, so Insiders who are currently on 9941 should be able to update their phones if they are already in the Fast ring. If they are in the Slow ring they will encounter the same issue with provisioning data. We apologize for the delay on this and are working to ensure it never happens again.

As I update this, I was able to download the Build 10051 update on my Lumia 730 running last preview Build 9941, but hitting the error “Update was downloaded but can’t be opened”.

Some new Windows Insiders (Lumia 635) are also able to get the update it seems but some are still facing the issue.

Bit of info here if you are facing issues while trying to update to the Windows 10 for Phones Technical Preview.

In case you are already on Build 9941 (First Preview), directly go to Settings–> Check for updates and you should be able to download the Build 10051.

In case you are new to Windows 10 for Phones preview you need to follow our tutorial and may have to wait for next few minutes for server error to be fixed.