When a build flow is started by another build with number greater than 1000, the link to the "parent" build flow is broken by inserting the "," (in our locale).
Example HTML code:
Started by build flow <a href="/job/project-jobs/job/SomeProject/job/some-buildflow/">project-jobs/SomeProject/some-buildflow</a> build number <a href="/job/project-jobs/job/SomeProject/job/some-buildflow/5,621">5,621</a>.
The comma in "5,621" within the href breaks the URL.
I was not sure which component is affects this issue, so I did a wild guess and also added "core".