This question has been answered by multiple community members. See the first response.
The first thing to try is logging out, deleting all files in the core/cache directory, and clearing your browser cache and cookies.
I know you said there wasn't anything in the MODX error log, but does anything show up in the Apache error log? Internal errors would not be logged by MODX, but they would be logged by Apache.
[Thu Apr 11 22:33:11 2013] [error] [client 187...] (12)Cannot allocate memory: couldn't create child process: /opt/suphp/sbin/suphp for /.../public_html/connectors/security/user.php, referer: http://www.<mysite>.com/manager/
[Thu Apr 11 22:33:11 2013] [error] [client 187...] File does not exist: /.../public_html/500.shtml, referer: http://www.<mysite>.com/manager/
[Thu Apr 11 22:33:11 2013] [error] [client 187...] (12)Cannot allocate memory: couldn't create child process: /opt/suphp/sbin/suphp for /.../public_html/connectors/resource/index.php, referer: http://www.<mysite>.com/manager/
[Thu Apr 11 22:33:11 2013] [error] [client 187...] (12)Cannot allocate memory: couldn't create child process: /opt/suphp/sbin/suphp for /.../public_html/connectors/resource/index.php, referer: http://www.<mysite>.com/manager/
128M should be more than enough, unless there's some kind of unterminated loop or recursion going on in those processors. But it points to a server issue - perhaps it's overloaded with too many accounts (which is not uncommon with shared hosts).
Are you running an opcode cache (Xcache, eAccelerator, APC)? Is suhosin installed?
In the MODX settings, are compress CSS/JS enabled? What about the concat_js setting? Try disabling all of those and see what happens.