Starting with release 3.04/01 ROOT has a new feature that allows printing stack traces for ROOT (and therefore also POOL) applications. When this features creates problems (because it requires the compiler option -lpthread and may hang up the program in case of errors if the option was not used), it can be turned off by setting in $ROOTSYS/etc/system.rootrc the variable:
Root.Stacktrace: no
You can also set this in ~/.rootrc or in ./.rootrc, depending on how globally you want to change this behaviour.