HTML error

Hi,
Checking my website at https://validator.w3.org for html errors gives me:

Error: Bad value https://www.facebook.com/sharer/sharer.php?s=100&p%5Burl%5D=http%3A%2F%2FSSSSSSSSS%2F&p%5Btitle%5D=Home+page for attribute href on element a: Illegal character in query: [ is not allowed.

At line 221, column 193 in resource http://SSSSSSSSS

Where SSSSSSSSS is my sitename, edited out for confidentiality.

I believe this URL is being generated by this plugin – -please address.