I've seen this issue where it has the www and takes out the www from the address- the problem is with the cookie settings in admin cp... make the cookie domain: (blank).
However this also points to a bigger issue that people can access your site both with and without the "www" - this is bad for SEO because it looks like 2 different sites to search engines. You should read up on .htaccess files and use it to forward one version to the other, doesn't matter which you choose to go with so long as you are consistent. If anyone enters it the 'wrong' way htaccess will forward them to the 'right' version.
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