Cannot create a space or a page in demo space

I am using a trial server Confluence 5.8.9.
I cannot create a new page if I login as admin; cannot create any pages in the predefined demo space either.
The Create Dialog has a spinner running indefinitely.
I found the stacktrace:

25-Aug-2015 00:04:47.653 SEVERE [http-nio-8901-exec-3] org.apache.catalina.core.StandardHostValve.custom Exception Processing ErrorPage[errorCode=500, location=/500page.jsp]
org.apache.jasper.JasperException: An exception occurred processing JSP page /500page.jsp at line 120

117:
118: if (sysInfoService != null)
119: {
120: confluenceInfo = sysInfoService.getConfluenceInfo();
121: memoryInfo = sysInfoService.getMemoryInfo();
122: dbInfo = sysInfoService.getSafeDatabaseInfo();
123: sysinfo = GeneralUtil.convertBeanToMap(sysInfoService.getSystemProperties());
...

Caused by: com.atlassian.confluence.tenant.VacantException: Confluence is vacant, a call to tenanted [public abstract net.sf.hibernate.Session net.sf.hibernate.SessionFactory.openSession() throws net.sf.hibernate.HibernateException] is not allowed.
at com.atlassian.confluence.tenant.TenantGate$1.lambda$create$413(TenantGate.java:45)
at com.atlassian.confluence.tenant.TenantGate$1$$Lambda$100/1513624464.invoke(Unknown Source)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:204)
at com.sun.proxy.$Proxy111.openSession(Unknown Source)
at sun.reflect.GeneratedMethodAccessor201.invoke(Unknown Source)