When a crash happens we send crash report to updatecheck.io which is hosted on render.com
See https://github.com/sumatrapdfreader/sumatrapdf/blob/master/src/CrashHandler.cpp#L30

It also happens when program encounters a condition that I think is a bug and want to gather more info about what happens, so that I can fix the issue. See ReportIf in the code.

Can you send me one of such epub files that trigger this so that I can investigate and fix the problem? See https://www.sumatrapdfreader.org/docs/How-to-submit-bug-reports

Related bug report: #2540