I'm having an odd login problem. At first I noticed that when I tried to log into the admin CP, the login page would simply refresh back to the login form. If I then refreshed the page through my browser, the admin CP would appear. But if I didn't, I'd just get a blank login form again.
Then, a user told me that she was having trouble logging in. She said the page refreshed but she still wasn't logged in.
I'm just gonna *ahem* step on in here and raise my hand as well. I just encountered the exact same problem and I'm not running the Scheduled Post add-on either. I started to panic a bit until I hit F5.
On that note, I've actually had to hit F5 a lot today to catch a few things up to speed. One moment, a private conversation message I send doesn't appear after returning to the conversation a minute later, and then after a quick cache clear, poof! it's there again. As if that wasn't enough, I check my alerts, mark them as read, and then return thirty minutes later to see the same alerts as before unread.
Have you disabled the caching?
What caching mechanism are you using?
I have found on mine that sometimes I have to restart the PHP processor - and sometimes on occasion even memcached. I use the PHP opcode caching and then also memcached with caching enabled in the config.php and Fetch Templates as Files in the ACP.