org.hibernate.dialect.HSQLDialect creates drop sequence statments like this:

"drop sequence " + sequenceName;

which leads to

2016-03-09 10:35:27.186 ERROR 6248 — [ restartedMain] org.hibernate.tool.hbm2ddl.SchemaExport : HHH000389: Unsuccessful: drop sequence <sequenceName>
2016-03-09 10:35:27.186 ERROR 6248 — [ restartedMain] org.hibernate.tool.hbm2ddl.SchemaExport : user lacks privilege or object not found: STEMPEL_SEQ in statement [drop sequence <sequenceName>]

This could be easily solved by generating the drop sequence as follows:

"drop sequence if exists " + sequenceName;

according to the hsqldb drop dequence docs