MongoDB Upgrade to 3.0: HostAndPort Exception

I want to upgrade my small MongoDB-Cluster (1 mongos, 3 config-servers, 2 shards a 3 nodes) running on OpenSuSe 13.1 (64bit) from 2.6.7 to 3.0.5. But it fails at the second step of the upgrade guide: upgrade a mongos instance

What I'm done:

• downloaded a legacy-linux-64bit-version of MongoDB 3.0.5
• deactivated load balancer

• tried to start mongos with

  mongos –configdb data/configdb –config config/router.conf –upgrade

This is the output of mongos:

2015-08-07T15:41:43.281+0200 W SHARDING running with 1 config server should be done only for testing purposes and is not recommended for production
2015-08-07T15:41:43.290+0200 I SHARDING [mongosMain] MongoS version 3.0.5 starting: pid=22726 port=27027 64-bit host=myhostname (--help for usage)
2015-08-07T15:41:43.290+0200 I CONTROL  [mongosMain] db version v3.0.5
2015-08-07T15:41:43.290+0200 I CONTROL  [mongosMain] git version: 8bc4ae20708dbb493cb09338d9e7be6698e4a3a3
2015-08-07T15:41:43.290+0200 I CONTROL  [mongosMain] build info: Linux build2.ny.cbi.10gen.cc 2.6.32-431.3.1.el6.x86_64 #1 SMP Fri Jan 3 21:39:27 UTC 2014 x86_64 BOOST_LIB_VERSION=1_49
2015-08-07T15:41:43.290+0200 I CONTROL  [mongosMain] allocator: tcmalloc
2015-08-07T15:41:43.290+0200 I CONTROL  [mongosMain] options: { config: "conf/router.conf", net: { bindIp: "myhostname", port: 27027 }, security: { keyFile: "res/mongodb.key" }, sharding: { configDB: "data/configdb/" }, upgrade: true }
uncaught DBException in mongos main:
9 Empty host component parsing HostAndPort from ""

I'm using the same settings as the already running instance, so the network settings should be correct.

Anyone a clue what I'm doing wrong here? Why is there a warning about one config server only, while I'm using three? And what does the exception mean?