Different LO versions had different default databases. Curently (v6.3.3.x) is HSQLDB. This is mainly due to the number of outstanding Firebird issues and the complexity to get around some of them.
Many different databases you can use do not require Java (with exception of Wizards and Report Builder) - Firebird, MySQL, PostgreSQL and many more.
It is questionable as to what you needed to install (definitely should not be HSQLDB - another could cause problems) since I have been using Base for years without any such problem. Wizards and Report Builder require the installation of a correct JAVA even with Firebird. I suspect you may have this problem if the JAVA situation wasn’t resolved.
There is always the possibility of some corruption in the User Profile. A clean install of LO does not renew/replace/refresh this. To clear see → LibreOffice user profile.