LibreOffice Mail merge crashes

Whenever I try to activate mail merge in LibreOffice, LibreOffice crashes. I tried to find bugs, guidance for an hour - but developed no clue.

Does anybody have an idea about how to identify the problem?

the system is 5.3.13-300.fc31.x86_64

And LibreOffice is:
Version: 6.3.3.2.0+
Build ID: 6.3.3.2-7.fc31
CPU threads: 8; OS: Linux 5.3; UI render: default; VCL: gtk3;
Locale: en-GB (en_GB.UTF-8); UI-Language: en-US
Calc: threaded