The server name bla is unknown to the administration server

If you get this:

weblogic.management.ManagementException: The server name bla is unknown to the administration server. Check if restart is required.
        at weblogic.management.provider.internal.RuntimeAccessImpl.initialize(RuntimeAccessImpl.java:447)
        at weblogic.management.provider.internal.RuntimeAccessService.start(RuntimeAccessService.java:49)
        at weblogic.t3.srvr.ServerServicesManager.startService(ServerServicesManager.java:462)
        at weblogic.t3.srvr.ServerServicesManager.startInStandbyState(ServerServicesManager.java:167)
        at weblogic.t3.srvr.T3Srvr.initializeStandby(T3Srvr.java:881)
        at weblogic.t3.srvr.T3Srvr.startup(T3Srvr.java:568)
        at weblogic.t3.srvr.T3Srvr.run(T3Srvr.java:469)
        at weblogic.Server.main(Server.java:71)

oracle documentation says:

http://docs.oracle.com/cd/E23943_01/apirefs.1111/e14397/Config.html

Cause The -Dweblogic.Name command line property specifies a server that does not exist in the configuration that is active on the administration server. This can occur if the managed server was added to the configuration along with non-dynamic changes that affect the administration server. In this case, the administration server must be restarted.

Action Check to see if the administration server is marked as needing a restart. If it is, then restart the administration server before trying to boot this managed server. Check the configuration for the list of configured servers.

Yet there is another explanation: that your managed server is connecting to an admin belonging to a different domain! Check that there are no processing listening on the ip/port that you have given as admin url to the weblogic server.