I run many WordPress installations on many hosts (30-40 installs on 4-5 different hosting environments). Everything runs fine, except one. Its on the host (1and1) I run the most installs on too. PHP info for the host.
This one install (taken from a wp.com blog) has always given Out of Memory errors when trying to do automatic upgrades. Recently the user permissions have gotten messed up. The main author on the account (an administrator) couldn't publish her own posts, but could save them for review and then publish the review. I (another admin account) can't create new users (nor can she), when I TRY to create a new user it works fine (WP even confirms it) but the user information is never entered into the tables.
We have since exported the blog, and rehosted it on WP.com and it works fine. I have tried taking the EXACT SAME export XML and importing it into a brand new WP install with a brand new database, and I get an out of memory error. We've tried exporting the posts from WP.com and putting it in a new install/database and it still happens. Obviously I've tried everything with no addons/plugins.
Only thing I can think is that its some malformed/unescaped string a post somewhere that's messing things up a bit.
Please let me know what additional information you need, or any suggestions you might have.
PS - I've tried increasing the WP memory limit in wp-config.php as well as php's global memory limits in php.ini (and confirmed in the phpinfo() that it was active).