There are many threads, plus one really long one at Google AdSense Help forums with publishers complaining they cannot login to the AdSense portal. I tried it myself and I have no problem, but it seems like the typical person complaining are those new to AdSense.
Most seem to be related to Google taking down the account for a manual "review." The issue is, Google said that the review would only take about a week. Many of these users say it has been over three weeks with the same issue and no word from Google on the status of that review.
It does seem like some mass review is going on. I have seen tons of complaints in the forums about publishers being terminated from the program and tons of threads like this that they cannot login to their accounts.
Some say that upon manual review, they can no longer login due to some cycle issue. One explained the process as, "login screen --> enter details --> click login --> back to login screen." That seems cookie related to me and likely can be fixed by deleting the Google cookie causing the issue.
Overall, there does seem to be a major review cycle going on right now.
Forum discussion at Google AdSense Help.