Check out this site (info site, NOT a shop!) - they've written an article on it that solved my problem with the R4i. http://www.r4dsrevolution.co.uk/r4i-system-date-error-problem-freeze They don't actually say WHY it happened but it seems to have happened to loads of people, where setting back the date on your DS console just fixes it.
My son's did this and I updated the software on the R4 website and it worked then but I had issues with saved games and new games so have added a thread in this forum to ask for help. I'm keeping my eyes peeled praying to the gods of AfterDawn to help...
I think there's been a fix to the issue - tried it on the R4i I have at home and it seems to have worked!! Blog post here: http://www.nds-gear.co.uk/blog/2010/05/nds-gear-solves-r4i-sdhc-system-date-error-problem/