Replace the line in htpasswd file:
Go to: http://www.htaccesstools.com/htpasswd-generator-windows/
(If the link is expired, search another generator from google.com.)
Enter your username and password. The site will generate an encrypted line. Copy that line and replace it with the previous line in the file "repo/htpasswd".
You might also need to Clear the 'Authentication data' from TortoiseSVN ? Settings ? Saved Data.