I had also the same problem. LibreOffice (6.3.4.2 snap1), installed by "Ubuntu Software" on Ubuntu 18.04 could not open documents attached to e-mails (Thunderbird) and even those on the web. It had also problems in linking with Zotero. I removed it completely and installed the one from the LibreOffice site (https://www.libreoffice.org/donate/dl/deb-x86_64/6.3.4/en-US/LibreOffice_6.3.4_Linux_x86-64_deb.tar.gz). LibreOffice now works without any problem, and it opens much faster.
I had also the same problem. LibreOffice (6.3.4.2 snap1), installed by "Ubuntu Software" on Ubuntu 18.04 could not open documents attached to e-mails (Thunderbird) and even those on the web. It had also problems in linking with Zotero. I removed it completely and installed the one from the LibreOffice site (https:/ /www.libreoffic e.org/donate/ dl/deb- x86_64/ 6.3.4/en- US/LibreOffice_ 6.3.4_Linux_ x86-64_ deb.tar. gz). LibreOffice now works without any problem, and it opens much faster.