A new Bibliography database was created when I installed 5.5.1 (may not be the right number because it was not in the version drop down menu) and 4.3.7.2 was auto-removed. I lost all my Bibliography references in the DB. This may need to be taken into account in the Installer. According to Alex Thurgood (in Bug 98665): Unfortunately a known and longstanding issue with installation packets. Each new version of LO replaces the built-in bibliography database (because it is hardcoded into the app). As a user, one needs to remember to save the bibliography database before installing a new version, and then copy it back to the appropriate place.
There is no LIbreOffice version 5.5.1 (at least not for a while yet). However, your report is a DUP of bug 76967, so I have re-opened that one. Please read the previous comments there, and provide any further information that you might have. *** This bug has been marked as a duplicate of bug 76967 ***