Ok, I turned on debug mode as I thought there might be an error, and there is, this is what it says:
Joomla!
500 - An error has occurred.
JDatabaseMySQL::query: 1104 - The SELECT would examine more than MAX_JOIN_SIZE rows; check your WHERE and use SET SQL_BIG_SELECTS=1 or SET SQL_MAX_JOIN_SIZE=# if the SELECT is okay SQL=SELECT u.id as user_id, u.email, u.username, u.registerDate, IFNULL(i.first_name,SUBSTRING(u.name,1,LOCATE(" ",u.name))) AS first_name, IFNULL(i.last_name,TRIM(SUBSTRING(u.name,IF(LOCATE(" ",u.name)=0,1,LOCATE(" ",u.name))))) AS last_name, COUNT(o.virtuemart_order_id) AS order_count FROM j7qm2_users u LEFT JOIN j7qm2_virtuemart_userinfos i ON i.virtuemart_user_id=u.id AND i.address_type="BT" LEFT JOIN j7qm2_awocoupon_plg_vm_email_register a ON a.user_id=u.id LEFT JOIN j7qm2_virtuemart_orders o ON o.virtuemart_user_id=i.virtuemart_user_id WHERE u.block=0 AND u.registerDate>="2012-12-18 00:00:00" AND a.user_id IS NULL GROUP BY u.id LIMIT 10
Return to Control Panel
Call stack
#    Function    Location
1    JAdministrator->render()    /htdocs/public/www/administrator/index.php:52
2    JApplication->triggerEvent()    /htdocs/public/www/administrator/includes/application.php:204
3    JDispatcher->trigger()    /htdocs/public/www/libraries/joomla/application/application.php:642
4    JEvent->update()    /htdocs/public/www/libraries/joomla/event/dispatcher.php:161
5    call_user_func_array()    /htdocs/public/www/libraries/joomla/event/event.php:71
6    plgSystemAwocouponEmail->onAfterRender()     
7    plgSystemAwocouponEmail->sendEmails()    /htdocs/public/www/plugins/system/awocouponEmail/awocouponEmail.php:71
8    plgSystemAwocouponEmail->sendRegistrationEmails()    /htdocs/public/www/plugins/system/awocouponEmail/awocouponEmail.php:113
9    JDatabase->loadObjectList()    /htdocs/public/www/plugins/system/awocouponEmail/awocouponEmail.php:193
10    JDatabaseMySQL->execute()    /htdocs/public/www/libraries/joomla/database/database.php:1100
11    JError::raiseError()    /htdocs/public/www/libraries/joomla/database/database/mysql.php:548
12    JError::raise()    /htdocs/public/www/libraries/joomla/error/error.php:251
Not really sure what to make of the error "The SELECT would examine more than MAX_JOIN_SIZE rows" as I have not seen it before.
Can you access the database and run the query itself?
SELECT u.id as user_id, u.email, u.username, u.registerDate, IFNULL(i.first_name,SUBSTRING(u.name,1,LOCATE(" ",u.name))) AS first_name, 
IFNULL(i.last_name,TRIM(SUBSTRING(u.name,IF(LOCATE(" ",u.name)=0,1,LOCATE(" ",u.name))))) AS last_name, 
COUNT(o.virtuemart_order_id) AS order_count
 FROM j7qm2_users u LEFT
 JOIN j7qm2_virtuemart_userinfos i ON i.virtuemart_user_id=u.id AND i.address_type="BT"
 LEFT JOIN j7qm2_awocoupon_plg_vm_email_register a ON a.user_id=u.id
 LEFT JOIN j7qm2_virtuemart_orders o ON o.virtuemart_user_id=i.virtuemart_user_id
 WHERE u.block=0 AND u.registerDate>="2012-12-18 00:00:00" AND a.user_id IS NULL GROUP BY u.id LIMIT 10
If that does not work on its own, try this:
SELECT u.id as user_id, u.email, u.username, u.registerDate, IFNULL(i.first_name,SUBSTRING(u.name,1,LOCATE(" ",u.name))) AS first_name, 
IFNULL(i.last_name,TRIM(SUBSTRING(u.name,IF(LOCATE(" ",u.name)=0,1,LOCATE(" ",u.name))))) AS last_name, 
COUNT(o.virtuemart_order_id) AS order_count
 FROM j7qm2_users u LEFT
 JOIN j7qm2_virtuemart_userinfos i ON i.virtuemart_user_id=u.id AND i.address_type="BT"
 LEFT JOIN j7qm2_awocoupon_plg_vm_email_register a ON a.user_id=u.id
 LEFT JOIN j7qm2_virtuemart_orders o ON o.virtuemart_user_id=u.id
 WHERE u.block=0 AND u.registerDate>="2012-12-18 00:00:00" AND a.user_id IS NULL GROUP BY u.id LIMIT 10
Does that work?  If not run the below query and then run the first 2 again.
SET SQL_BIG_SELECTS=1;
Does it work after running that?