With the upgrade of Piwik 1.7.x to 1.8x, the automatic reports don't work anymore.
However generating manualy the pdf works (and sending it by mail).
The problem seems to affect fresh installation too :
Update : activating auto archiving in 1.8.2 resolves the problem
I propose that we set this ticket as wont fix and write a FAQ.
Setting up the cron is required for accurate daily scheduled reporting.