the Programm "Writer" crashes, when you use the Mail Merge Wizard (German "Serien-Mail Assistent") for E-Mail.
If in Nummer "4.Create salutation" you left the field "Field Name" blank, then if you at "8. Save, print or send" the Program crash, at "Send merged Document as E-Mail" if you click at in the field "Properties..." on "OK".
Could you try to get a backtrace? https://wiki.documentfoundation.org/How_to_get_a_backtrace_with_WinDbg
Maybe test with 5.2 first: http://www.libreoffice.org/download/pre-releases/
Set to NEEDINFO.
Change back to UNCONFIRMED, if the problem persists. Change to RESOLVED WORKSFORME, if the problem went away in 5.2.
No feedback after 6 months, let's put this one to INSUFFICIENT DATA.
Florian: don't hesitate to reopen this tracker if you still reproduce this with 5.3.0 version.