The format you show is from your editor rather than TeX however the original TeX warning message will be in the log.
By default it will just show the box contents as [] but if you put
\showboxbreadth=50
\showboxdepth=50
Near the top of the file it will show a representation of the box contents (one line per character, so can be quite verbose, but if you need to see more, increase 50, or use \maxdimen)
As it is in the output routine the error doesn't directly relate to a line in your file but to a failure by Tex to make up a page correctly.
Overfull boxes in the output routine are usually related to the page head, for example trying to fit a two line chapter title into a page heading that only takes one line.
But as it is underfull I would guess that it is the page body that is underfull. The usual way to get that is to have \textheight that is not exactly \topskip plus a multiple of \baselineskip then if there is no stretchy white space on the page to compensate, TeX can not fill the page, the page is short but can not stretch, but adding one line would make it over-full. In such a case TeX chooses to leave the page short but make a warning.
badbox lineis not a tex error message but is presumably the message filtered through your editor. "in the output routine" means (usually) the problem is in the page head, which may be using text from your section headings in some classes. – David Carlisle Feb 23 '15 at 12:37.logfile. If the "Underfull/Overfull box" is warned then the contents of the box is listed in the.logfile. – wipet Feb 23 '15 at 12:57\showboxbreadth=50\showboxdepth=50and then he/she can read my first comment here. – wipet Feb 23 '15 at 13:39