This question has been answered by ilardo. See the first response.
It could be a problem with one of the dashboard widgets or (less likely) a CMP.
For starters, I would empty this table (but don't delete the table itself):
modx_dashboard_widget_placement
That shouldn't delete any widgets, it will just remove them from any dashboards.
Delete all files in the core/cache directory before trying to log in.
I am sure you did this, but after following Bobs suggestion did you manually empty the core/cache folder again?
What does your PHP/Apache error log have to say about the 500 error?
It could be a problem with one of the dashboard widgets or (less likely) a CMP.
For starters, I would empty this table (but don't delete the table itself):
modx_dashboard_widget_placement
That shouldn't delete any widgets, it will just remove them from any dashboards.
Delete all files in the core/cache directory before trying to log in.