In fact this is not a question because I've already found the solution but I've strugguled so long with it that I'd rather share this one with you than let it die in the depth of my mind
This problem occured to me on an existing project that was using maven assembly 2.2 for some time
The whole exception message was :
Failed to execute goal org.apache.maven.plugins:maven-assembly-plugin:2.2:assembly (default-cli) on project test: Failed to create assembly: Error creating assembly archive jar-with-dependencies: Problem creating jar: no entry name specified
There was no more message even with a mvn -X
At the beginnig I thought it was due to a bad url like :
jar:file://MY_FILE.jar!
But all this was caused by an hidden file named ".#log4j.xml..." which was automatically created by CVS during a conflict resolution
==> solution is resolve the conflict and remove the file
I've filed a bug report on this : PLXCOMP-169
As I said this is not a bug and will need an improvment from the plexus team.
Until then you will need to find by yourselves ".#" files in your project AND dependencies (this one was in a project of mine which was a dependency to the project beeing assembled).
Thanks to Jon to point me out the fact that an accepted answer would be more intuitive.