First of all, welcome to the forum!
You experience this issue because incorrect Pligg site URLs were added to the software. As explained in user guide and help section, you should add MAIN URL of Pligg site. For example, if Pligg site is available at
http://some_pligg_site.com, you should add this URL, and NOT
http://some_pligg_site.com/register.phpAutomatic Pligg Submitter parses correct registration, login and submission links automatically, so you should always enter the main URL only.
Please try removing Pligg site links from your installation and re-adding them again (be sure to use only main URLs now). As soon as you do this, all accounts should be registered successfully.