Hi Kai. Are you using the org.hibernate.dialect.H2Dialect dialect?
Because usually with this dialect the constraints are add added in the subsequent alter table rows.
Such as:

code
create table Client (
id integer generated by default as identity,
name varchar(255),
primary key (id)
)
create table User_ (
id integer generated by default as identity,
credentials_id bigint not null,
functionrights_id bigint not null,
name varchar(255) not null,
client_id integer not null,
primary key (id)
)
alter table User_
add constraint UKk72lpbblgi5qbf010ipy3ec4o unique (client_id, name)
alter table User_
add constraint UK_icj8srimshmevc7ym1xbfpb6w unique (credentials_id)
alter table User_
add constraint UK_k6xve0c683wrflmclha4qby86 unique (functionrights_id)
alter table User_
add constraint FKl461fnjx3swni7l0aiyi8rd0k
foreign key (client_id)
references Client
code