Sorry to hear you are hitting up against problems, magmazing.
Can you confirm if the reverse situation is true – ie with all other plugins deactivated and only a default, unmodified theme running – that there is still a problem on the Appearance → Widgets admin screen?
I wonder if you are essentially facing an issue with lack of memory: in the first instance can you try increasing the memory limit by adding the following to your wp-config.php file:
define( 'WP_MEMORY_LIMIT', '96M' );
If that doesn’t work the next step is to see if there are any entries in the server error logs corresponding to these ‘crashes’ (seek help from your hosting provider if you are unsure how to access the logs).