This time I know for sure: I created a new document, a table, added some data in the table cells, but I want to merge 2 cells on a column. I can merge 2 cells on a column, but if I want to merge the next 2 cells from the next column, Writer crashes. I chose not to recover, I open my document and I can do a single merge, cause at the next merge of the cells, Writer crashes again. And so on. Does someone else has this problem too?
Cannot reproduce. You may want to edit the question to give more precise instructions. But it shouldn't crash, so it sounds like there's something wrong.
According to the website, version 6.0.3 is for the "technology enthusiast, early adopter or power user". To me, that means, Expect bugs. Personally, I would wait for a more stable version before reporting such a bug.
However, the developers may find it helpful if you report potential bugs that have not been reported yet. https://wiki.documentfoundation.org/QA/BugReport.