unexpected error on Install Plugins page

Whenever I goes to install plugins (search) page on WordPress installation the page comes with a message
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An unexpected error occurred. Something may be wrong with www.ads-software.com or this server’s configuration. If you continue to have problems, please try the support forums.
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even then I tried to install new plugin, the error page reflects the same message as above

Said problem spreads on all WordPress installations on my VPS, either new plugins/ changes done or not.

Although all website are updated WP and plugins
but sucuri.net scanner points out “WP is outdated” !!!

and there is no update option to WP 3.7

there is something wrong , as I feel site’s loading performance is going very slow since a week onward

am unable to detect the source of bad smell ??

there was another problem too
RSS feed validator plugin said error
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Warning: strpos() [function.strpos]: Offset not contained in string in /home/hosthind/public_html/XXXXXXXXX.com/wp-content/plugins/rss-feed-validator/RSS-Feed-Checker.php on line 43
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and a point to note
without doing any changes, all of sudden, flood of spam comments now reflecting in comment section of all WP installation
before this, Akismet took care of all this matter

feeling helplessness