We used to build IzPack installer in an Eclipse environment using the IzPack 4.3.5 Ant Task. After upgrading IzPack to 5.1.3 the build fails with URL/URI related errors and NPEs.
I found out that in our case the problems where caused by whitespaces in some of the relevant paths. The build fails if either
1. the Eclipse program directory,
2. the Eclipse workspace directory,
3. the Eclipse project name/directory,
4. the IzPack libs directory, or
5. the JDK directory
contain any chars that are not valid in a URL/have to be URL encoded. Please find attached a primitive patch against IzPack 5.1.4-SNAPSHOT-f390e that at least works for me (but may not be correct, I simply don't have enough knowledge about the IzPack internals).