Method 1 (if error appears when opening website builder from WHMCS or WHM being logged in as admin):
Take the following steps to fix this issue:

• Login to cPanel server per SSH;
• Delete file /home/[user]/.spbldr_localStorage (this file is created by website builder plugin);
  Note: If you want to delete this file through cPanel file manager initially you may not see files starting with "." by default. To fix this, click "Settings" in top right corner of file manager page and check option "Show hidden files (dotfiles)".
• Delete FTP accounts of type "a1b2c3d4@domain.com" (they were created by website builder plugin);
• Open website builder from WHMCS (or WHM) again and publish website to check if the problem was solved.

Method 2 (if error appears when opening website builder from customer cPanel account being logged in with customer's username and password):

• Login to WHM;
• Find customer with problem on "List accounts" page;
• Change customer's password (you can set the same password as it was earlier);
• Open website builder from customer's cPanel account again and publish website to check if this solved the problem.