I found that debugging Aegir issues was a lot easier if you executed the Drush commands directly.
You can find the command by looking through the Log Messages keeping an eye out for entries like
Backend invoke: /usr/bin/php -d magic_quotes_gpc=Off -d magic_quotes_runti... (Expand)

Below entries like the above you will find the command that was executed, expand that by clicking the (Expand) and copy the command (I found sometimes the expand did not work, in cases like this I just use chrome tools to look at the source and copy it)
/usr/bin/php -d magic_quotes_gpc=Off -d magic_quotes_runtime=Off -d magic_quotes_sybase=Off /var/aegir/.composer/vendor/drush/drush/drush.php --php=/usr/bin/php --php-options=' -d magic_quotes_gpc=Off -d magic_quotes_runtime=Off -d magic_quotes_sybase=Off' --backend=2 --yes @centosintranet provision-install-backend --client_email='this.email@is.invalid' 2>&1
All you need to do now is run this as the Aegir user via bash. If you use the apt-get package to install Aegir you should be able to open a terminal and enter
sudo su - aegir -s /bin/bash
You will be prompted for your password, after entering it you will have a bash shell as the Aegir user. Paste in the above drush command and press enter.
Watch it execute, I found it easier to work out what was going on by removing the --backend=2 option. If you still can't see what the issue is try adding -vvv or --debug to the drush command.
Using this method I have solves all issues I have run into thus far when Aegir falls over.