I've no idea why it's assigning 10 instead of 6 unless `credits` is already 4 as you're adding the two together. It just executes the query as you've supplied it. However you're missing a lot of proper quotes usage. Your query should be formatted as follows.
Code:
UPDATE `#__paidsystem_credits` SET `credits` = ( `credits` + 6 ) WHERE `user_id` = '[user_id]'
Quoting the user id will avoid SQL errors (which maybe the cause of your blank pages) when there's no user id. Beyond that I don't know what to suggest as your install is quite far behind (we're on CB 2.0.8 and CBSubs 4.0.0 RC2 now).