I am also experiencing problems with the 'show last X'. The default for the board is set to 30-days. I have reset this...no difference. My personal default is also 30 days and have reset my profile. I am using the new accelerator.
So far, it only seems to affect some forums, but not all. You can also set it to "Show Topics from last 30 days" on the forum page, but the next time you view that forum it will have changed itself to "Show topics from last day".
