I can confirm his issue. My site validates perfectly fine in W3C with CB login 4.1 disabled. As soon as I activate it, I get these errors. It most certainly references this module as the trigger.
Validation Output: 6 Errors
1. Error Line 225, Column 80: attributes construct error
…sms379/component/comprofiler/login" method="post" id="mod_loginform" class="cb…
✉
2. Error Line 225, Column 80: Couldn't find end of Start Tag form line 225
…sms379/component/comprofiler/login" method="post" id="mod_loginform" class="cb…
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3. Error Line 241, Column 15: Opening and ending tag mismatch: div line 223 and form
</table></form> </div>
✉
4. Error Line 248, Column 6: Opening and ending tag mismatch: body line 214 and div
</div>
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5. Error Line 815, Column 8: Opening and ending tag mismatch: html line 2 and body
</body>
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6. Error Line 816: Extra content at the end of the document
</html>