Haven't used IMa2 yet, but in IM, each allele should be on it's own
line with its own name. Forget about individuals, the unit of analysis
is the allele. It seems that you are grouping both alleles under one
name which is the problem.
One individual should be coded like:
indiv001_a AAATTCTCC...
indiv002_b AAAATCTCG...
etc.
Hope that solves your problem. Any little error in the input file can
cause the program to crash.
Andy