[Flow] using 3rd party packages

[Sasky, Patrick]

Hey there,

I have installed Respect.Rest via composer and activated this via ./flow 
package:activate Respect.Rest

./flow package:list outputs that this package is active.

But when I start to use this package with use Respect\Rest\Router; an 
error occurs:
Execution of subprocess failed with exit code 255 without any further 
output.

There is no detailled error in logfiles and also in Data/Logs/Exceptions 
is no useful information.

Can anybody help? I didn't get it..

Thanks!