Issue solved!
It appears that the problem was because the message mailconfirm.mail.body contained an apostrophe somewhere after {0} and between {1}. After replaced doesn't with does not it fixed the problem. I didn’t know apostrophes can’t be used in there.
P.S. Is it a bug or just my mistake and apostrophes should be escaped?
mailconfirm.mail.body=<html><body><h3 style="margin: 0 0 1em;">Hi, {0}!</h3>\
To confirm your email address, click on the confirmation link given bellow. If clicking on the link doesn't work, copy and paste the link in a new browser tab. <br /><br />\
<a href="http://www.domain.com/confirm_email.html?action=activate&hash={1}">http://www.domain.com/confirm_email.html?action=activate&hash={1}</a><br /><br />\
Kind regards,<br />\
Your Something
</body></html>
One doesn't took me about an hour to figure it out and push a fix. Hahaha.. From now on I consider apostrophes being evil!