[andrey@centos1 cli]$ ./membase server-list -c 127.0.0.1:8091 --debug
INFO: running command: server-list
METHOD: GET
PARAMS:  {}
ENCODED_PARAMS:
REST CMD: GET /pools/default
response.status: 200
ERROR: command: server-list: 127.0.0.1:8091, 'otpNode'

The reason you got this error is because you did not specify the username and password for you cluster. Below I repeated what you did and got the same result, but in the next example I added the username and password of my cluster and got the correct response. Please post again if this doesn't work for you.

[root@mike_1 ~]# /opt/membase/bin/cli/membase server-list -c 127.0.0.1:8091 --debug
INFO: running command: server-list
METHOD: GET
PARAMS:  {}
ENCODED_PARAMS:
REST CMD: GET /pools/default
response.status: 200
ERROR: command: server-list: 127.0.0.1:8091, 'otpNode'

[root@mike_1 ~]# /opt/membase/bin/cli/membase server-list -c 127.0.0.1:8091 -u Administrator -p password --debug
INFO: running command: server-list
METHOD: GET
PARAMS:  {}
ENCODED_PARAMS:
REST CMD: GET /pools/default
response.status: 200
ns_1@127.0.0.1 127.0.0.1:8091 healthy active