Pkcon consistently fails with message:

$ sudo pkcon update   
Updating system               [=========================]         
Waiting for authentication    [=========================]         
Starting                      [=========================]         
Querying                      [=========================]         
Resolving dependencies        [=========================]         
Installing updates            [=========================]         
Installing packages           [=========================]         
The transaction failed: internal-error, Installation aborted by user

and (!!!) 0 (zero) exit code (weird!). 

Reboot, reinstall did not help. -v does not help much, logs do not contain anything except activation of the PackageKit service.

$ sudo pkcon -v update
22:15:38        PackageKit          Verbose debugging enabled (on console 1)
22:15:38        PackageKit          notify::connected                                                                                         
22:15:38        PackageKit          notify::backend-description                                                                               
22:15:38        PackageKit          notify::version-major                                                                                     
22:15:38        PackageKit          notify::groups                                                                                            
22:15:38        PackageKit          notify::backend-author                                                                                    
22:15:38        PackageKit          notify::backend-name                                                                                      
22:15:38        PackageKit          notify::version-micro                                                                                     
22:15:38        PackageKit          notify::roles                                                                                             
22:15:38        PackageKit          notify::mime-types                                                                                        
22:15:38        PackageKit          notify::filters                                                                                           
22:15:38        PackageKit          notify::version-minor                                                                                     
22:15:38        PackageKit          notify::network-state                                                                                     
22:15:38        PackageKit          notify::distro-id                                                                                         
22:15:38        PackageKit          filter=(null), filters=0                                                                                  
22:15:38        PackageKit          adding state 0x806f448                                                                                    
22:15:38        PackageKit          role now update-system                                                                                    
Updating system               [                         ] (0%)  22:15:38        PackageKit          already processing request 0x1, so ignoring                                                                                                                                             
22:15:38        PackageKit          already processing request 0x1, so ignoring                                                               
22:15:38        PackageKit          already processing request 0x1, so ignoring                                                               
22:15:38        PackageKit          emit transaction-list-changed (when idle)                                                                 
                              [=========================]                                                                                     
Waiting in queue              [                         ] (0%)  22:15:38        PackageKit          already processing request 0x2, so ignoring                                                                                                                                             
                              [=========================]                                                                                     
Starting                      [=========================]         
Querying                      [=========================]         
Resolving dependencies        [=========================]         
Installing updates            [=========================]         
Installing packages           [                         ] (0%)  22:15:48        PackageKit          emit transaction-list-changed (when idle)
22:15:48        PackageKit          remove state 0x806f448                                                                                    
                              [=========================]                                                                                     
The transaction failed: internal-error, Installation aborted by user