Magento 1.9 Extensions – Service Temporarily Unavailable After Installing a Module

I'm running NGinx 1.6.2 with Magento 1.9.1.1 – just wanted to install SMTP Pro Email via the Connect/Mag Downloader webinterface and it got till:

Checking dependencies of packages
Starting to download ASchroder_SMTPPro-2.0.6.tgz …
…done: 32,911 bytes

Then I checked the frontpage after a couple of minutes because I thought there was a problem and I get now everywhere:

Service Temporarily Unavailable
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.

There are no logfiles written (/var/log/ is empty) and I don't see anything in the NGinx error – just a 200 in the NGinx access log.

I checked all directories for leftovers from the SMTP extension but nothing has actually been written so far.
I emptied the cache in /var/cache but nothing changes either.

My question: What happens before the download of an extension finished or is being extracted?
Maybe via that way I could figure out how to get my shop back?
Or does anybody know where to digg?