Further testing has been made.
When I use CBSubs > Basket > Pay button, I get the same result. It will not update the #__users username field.
I did change the query so that the update would take place in a different table and that worked perfectly so the SQL action is executing and the query seems to do what it should.

Is there anything else that I could try to make this work?

I just did a test, paying via paypal. There were no differences, username was not updated.

I did "register new member" on frontend, added all the mandatory fields and then did the payment via paypal. did not work.

we are onto something here. After changing to debug level I got this:

JDatabaseMySQL::query: 1093 - You can't specify target table 'ducati_users' for update in FROM clause SQL=UPDATE `ducati_users` SET `username` = ( ( SELECT `username` FROM `ducati_users` WHERE `username` >= 1 ORDER BY `username` DESC LIMIT 1 ) + 1 ) WHERE `id` = 6675

Maybe the only way of getting round this is to post the new username to a temporary table and then use a secondary SQL statement that updates users table with the new content from the temp table... or is it possible to do something else?