Upon opening LibreOffice, notification of extension update appeared in menu (green down-arrow to drive icon). After clicking on it, LibreOffice did not show window for update until 30 seconds had passed, and that window was blank. Trying to close the window or quit the application failed. Did hard shutdown.
This is a known bug. Now to find the original report.
*** This bug has been marked as a duplicate of bug 70607 ***