Thought this may be useful:

I got an error today about the site "not accepting a security token" and not letting me access certain parts of the site (User CP) as well as intermittently kicking me out on various threads. It was probably an error with Daylight Savings Time yesterday, which automatically corrected on my computer/phone, but seems to have failed to correct on the server somehow and would refuse to let me access anything requiring the "security token".

This was frustrating because I figured if I could change the DST settings in my User CP it would all be fixed, but it would not let me get into the settings by clicking the link. Eventually I found that by clicking on my "Private Messages" link, it would let me access the page that changes the DST options, and I no longer have problems.