Having the same issue. Found the limit is 50 line items. Once you go to 51, it will allow you, and seem to be working until you go to save the quote. It then truncates the quote to 50 items, and drops the additional items.
Started looking for the code limitation in the modules folder, but haven’t found it yet. Any help would be appreciated.
I would start by checking your web server log (typically called php_errors.log) to see how it’s breaking. Is it a memory limit exceeded? Is it an “array out of bounds” error? Is it time loading the page timeout?