Error on database launch (SEVERE existing object in cache) then "java.lang.OutOfMemoryError: Java heap space" errors

I have a pretty big database running on HSQLDB 2.5.0, using LibreOffice Base as the front-end. Everything was working great when I left on Friday night. Now, however, when the database launches I get this error:

SEVERE existing object in cache

And then I get a bunch of “java.lang.OutOfMemoryError: Java heap space” errors whenever a specific table is accessed.

Any suggestions on how to fix this?