WordPress overrides PHP's memory limit to 256M, with the assumption that whatever it was set to before is going to be too low to render the dashboard. You can override this by defining WP_MAX_MEMORY_LIMIT
in wp-config.php
:
define( 'WP_MAX_MEMORY_LIMIT' , '512M' );
I agree with DanFromGermany, 256M is really a lot of memory for rendering a dashboard page. Changing the memory limit is really putting a bandage on the problem.