I had the same problem. Four migration scripts set 0 as a default value for ForeignKey, which seems not accepted. Instead, you can set None as the default value. The following changes in the migrations script should fix this problem:
• go to the askbot/migrations directory
sed -i.old 's|$ForeignKey.*$default=0|\1default=None|g' 0055_*.py 0080_*.py 0093_*.py 0099_*.py