GMAJ error: Exception in thread "main" java.lang.ClassCastException: java.lang.StringBuffer cannot be cast to java.lang.String
It's a programming error that should look like this:

StringBuffer sb = (...)
(...)
String s = (String)sb;


while it should be

StringBuffer sb = (...)
(...)
String s = sb.toString();


( see also https://www.biostars.org/p/177992/ ) . You cannot do anything about it but asking the authors of the software.