Solution 1: The easiest solution is to clear the cache, try to login from different browser or incognito/private browser mode.

Solution 2: If first solution doesn't work, it is necessary to deactivate plugin manually. Please complete the following steps:

  1. Go to the /wp-content/plugins folder via FTP (http://codex.wordpress.org/FTP_Clients);
  2.  Find and rename “webarx” folder into “webarx1”;
  3.  After you log into your dashboard, you can rename the folder back from "webarx1" to "webarx" and disable reCaptcha from WebARX plugin settings.
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