debugging a single tenant configuration doesn't work

I've started to test a tenant installation.
The RTC client can be started with "server-nav05:7278/CronusNL900Demo/DEFAULT".
The object database is called "Cronus NL900 Tenant App".
The Database containing the company data is called "Cronus NL900 Tenant Demo"
"Cronus NL900 Tenant Demo" can't be opened through the developer environment, But the "Cronus NL900 Tenant App" can, because it contains the objects.
Now I want to debug and I open the debugger, but something goes wrong there.
When it tries to open the debugger, or at least that's what I expect, the standard client login screen opens, telling me it can't connect to "server-nav05:7278/CronusNL900Demo"

adding "/DEFAULT" doesn't help.
What could be wrong here?

I've checked the ClientuserSetting.Config and it consists of:
<configuration>
<appSettings>
<add key="Server" value="server-nav05" />
<add key="ClientServicesPort" value="7278" />
<add key="ServerInstance" value="CronusNL900Demo" />
<add key="TenantID" value="DEFAULT" />
...
<add key="UnknownSpnHint" value="(net.tcp://server-nav05:7278/CronusNL900Demo/Service)=Spn;" />
<add key="UrlHistory" value="server-nav05:7278/CronusNL900Demo" />
<add key="ServerPort" value="7278" />

Can you please help me out here.