Error starting activator


I am at a friend of mine and we migrated his openHAB 1.x to 2.3 a few weeks back.
Its running on raspbian.

We now wanted to install the mqtt binding, but it does not work.

So far the installation is very small.
Only ntp, mail and owserver2 are installed via config files.

The problem seems to be this WARN:

2018-07-18 23:54:01.842 [WARN ] [raf.features.internal.osgi.Activator] - Error starting activator                      Unexpected end of input at 1:1                                                                            
        at org.apache.karaf.features.internal.util.JsonReader.error( [10:org.apache.karaf.features.core:4.1
        at org.apache.karaf.features.internal.util.JsonReader.expected( [10:org.apache.karaf.features.core:
        at org.apache.karaf.features.internal.util.JsonReader.readValue( [10:org.apache.karaf.features.core:
        at org.apache.karaf.features.internal.util.JsonReader.parse( [10:org.apache.karaf.features.core:4.1.
        at [10:org.apache.karaf.features.core:4.1.5
        at org.apache.karaf.features.internal.region.DigraphHelper.readDigraph( [10:org.apache.karaf.feat
        at org.apache.karaf.features.internal.region.DigraphHelper.loadDigraph( [10:org.apache.karaf.feat
        at org.apache.karaf.features.internal.osgi.Activator.doStart( [10:org.apache.karaf.features.core:4.1
        at [10:org.apache.karaf.features.core:4.1.5]   
        at java.util.concurrent.Executors$ [?:?]                                       
        at [?:?]                                                      
        at java.util.concurrent.ThreadPoolExecutor.runWorker( [?:?]                               
        at java.util.concurrent.ThreadPoolExecutor$ [?:?]                               
        at [?:?]                                                                         

In the karaf console we cannot use the feature command which at least works on my v2.1 installation.
Paper UI does not show the Addons menu.

We cleared /var/lib/openhab2/cache and /var/lib/openhab2/tmp.
We deleted the json-Files wich were empty anyway.

Any idea what might be the cause for this issue?