Couchbase Authentication error

Hi,
I have set up a fresh couchbase server in VirtualBox (Community Edition, 5.1.1), but got a problem authenticating.

I tried to authenticate via the nodeJS SDK (cluster.authenticate), but always get an invalid username/password combination.

Then I checked with sdk-doctor and get an error on fetching the config, but everything else is working fine:
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| _
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Note: Diagnostics can only provide accurate results when your cluster
is in a stable state. Active rebalancing and other cluster configuration
changes can cause the output of the doctor to be inconsistent or in the
worst cases, completely incorrect.

18:20:00.967 INFO :arrow_forward: Parsing connection string couchbase://192.168.56.101/users
18:20:00.972 INFO :arrow_forward: Connection string identifies the following CCCP endpoints:
18:20:00.972 INFO :arrow_forward: 1. 192.168.56.101:11210
18:20:00.972 INFO :arrow_forward: Connection string identifies the following HTTP endpoints:
18:20:00.972 INFO :arrow_forward: 1. 192.168.56.101:8091
18:20:00.972 INFO :arrow_forward: Connection string specifies bucket users
18:20:00.972 WARN :arrow_forward: Your connection string specifies only a single host. You should consider adding additional static nodes from your cluster to this list to improve your applications fault-tolerance
18:20:00.972 INFO :arrow_forward: Performing DNS lookup for host 192.168.56.101
18:20:00.972 INFO :arrow_forward: Attempting to connect to cluster via CCCP
18:20:00.972 INFO :arrow_forward: Attempting to fetch config via cccp from 192.168.56.101:11210
18:20:00.981 INFO :arrow_forward: Selected the following network type: default
18:20:00.981 INFO :arrow_forward: Identified the following nodes:
18:20:00.981 INFO :arrow_forward: [0] 192.168.56.101
18:20:00.981 INFO :arrow_forward: mgmt: 8091, capi: 8092, projector: 9999
18:20:00.981 INFO :arrow_forward: kv: 11210, moxi: 11211
18:20:00.981 INFO :arrow_forward: Fetching config from 192.168.56.101:8091
18:20:00.992 INFO :arrow_forward: Failed to retreive cluster information (status code: 401)
18:20:00.993 INFO :arrow_forward: Successfully connected to KV service at 192.168.56.101:11210
18:20:01.008 INFO :arrow_forward: Successfully connected to MGMT service at 192.168.56.101:8091
18:20:01.057 INFO :arrow_forward: Successfully connected to CAPI service at 192.168.56.101:8092
18:20:01.061 INFO :arrow_forward: Memd Nop Pinged 192.168.56.101:11210 10 times, 0 errors, 0ms min, 0ms max, 0ms mean
18:20:01.061 INFO :arrow_forward: Diagnostics completed

Summary:
[WARN] Your connection string specifies only a single host. You should consider adding additional static nodes from your cluster to this list to improve your applications fault-tolerance

The user (users) got all rights including Admin and Bucket Full Access and if I change the password on the sdk-doctor just for checking if the password is correct, I get an additional (error: invalid bucket name/password) error. So the username / password combination is correct.

Do you got any hints on what might be the issue here?

Thanks for your help!
Jens