I like to use the user registration demo. I just cloned the project and compiled and started it, just out of the box. the following error occured:
'''HSQLDB connection error java.sql.SQLTransientConnectionException: connection exception: connection failure: java.lang.OutOfMemoryError: Java heap space connection exception: connection failure: java.lang.OutOfMemoryError: Java heap space'''
It looks like this exception happens when port 9010 used by HSQL by default is occupied by another application (don't ask how occupied port is connected to "outofmemory", no idea).
As a workaround, you can change HSQLDB connection port in the main data store settings.
More discussion here: https://www.cuba-platform.com/discuss/t/hsqldb-connection-error-outofmemory/13294/7