That error page doesn't tell much, could in the future click on the 'Details' link at the bottom of that page for more details.
At those times of the day there are number of 'maintenance' tasks running, e.g. backup, compiling stats, updating PuntingWiki, etc. When some of those tasks (e.g. backup) run they may hang the site for a minute or two.
Sometimes when I ban members it brings the whole site down for a few minutes, this is a known issue with the SMF forum software, the developers say it only affects large forums so don't have plans to fix this problem (it's related to MySQL indexes). If that's the case it's only going to get worse as we're racing to hit 2 million posts and 200,000 members.