I don't understand how a b2evo update can produce this error. I would understand that a PHP update would produce this error.

No matter what, I recommend you disable trackbacks:
http://b2evolution.net/man/comment-feedback-options

Thanks,
but disabling trackbacks isn't a solution for me.

This error was in the version before too. In version 6.4.2 - and is after the update too.

Errm,
this error is in version 6.6.3 - sorry for my mistake.

We'll fix it of course.

However I don't think we'll keep trackback support forever. It's more or less commonly accepted as being a broken obsolete design. Can you make a compelling case for why you absolutely need it?

@fplanque
Sure - there are some websites, which makes some weekly actions. There will be a theme/title you write about and all participating blogger will trackback this.

Another case: I make trackback to other posts I have made in the past so if ppl will read this older post - via google or so - will see the newer one in the trackback.

Trackbacks are old - but not obsolete.

Thanks.

Here is the fix you can apply right now to make it work:
https://github.com/b2evolution/b2evolution/commit/19c7a30fcbe74cecaa85d3f95ce2ba4f7aba7548