I guess the link is returned by line:

buf.append(DisplayURLProvider.get().getRunURL(build));

https://github.com/jenkinsci/mailer-plugin/blob/master/src/main/java/hudson/tasks/MailSender.java#L252

But I annot find the implementation for the "changes" url ,except in blue ocean code, where it looks right.

I suspect the changes for i18n in version 1.18 of the mailer-plugin.
I noticed, that the link in my mails ends with /Änderungen: (german localization).

So I think it is the MailSender.FailureMail.Changes property from mailer-plugin/src/main/resources/jenkins/plugins/mailer/tasks/i18n/Messages<locale>.properties which is appended to the job URL.

arbeita: do you have a colon appended to your link, which wasn't there in older mails?

Yes, in my mail is a colon after word "Changes". So it is the possible fault.

This is probably resolved after changes in https://issues.jenkins-ci.org/browse/JENKINS-40224

Could you evaluate against mailer-1.19?

Yes it works now. It use a complete different link with redirect in it, but it works.

Can you mark this as resolved then?

since update to mail plugin 1.19 it works