Actually there's the message after the installation:

"Now start Ajenti with 'service ajenti restart'" :)

Sorry about that, but we can't reliably restart it with Debian packaging system.

Why is that so? Because the package post-installation script is being run before python-support triggers are run. And before they are run, the application's python files may not yet be placed in the python dist-packages dir :(

Can you reproduce it? If so, could you also attach output of netstat... (see below) before and after Apache restart?

Looks like the port was already taken.

This looks like Ajenti port being taken by other application.

Could you please check with 'netstat -tlnp | grep <port>' once the panel becomes unavailable again?

I have just re-checked with 1.2.6 and it's working. Could it be something else? Could you make sure (by using 'sudo pdbedit -L') that user is indeed not being added?

Glad you figured it out :)

Fix committed, to be released in next build.