Multiple possibilities. Could be bad block data, unstable CPU, running out of RAM, some unknown bug.
You should create your own thread and post armorylog.txt and dbLog.txt. This thread is too long to track this issue here.
The problem got solved. While armory was stuck at some percentage everytime (from deleting the DB completely from armory folder), what i did was, killed the armory process and ArmoryDB as well. Restarted it and it started syncing properly with bitcoind.
So the above problem only happens when deleting the database folder in armory folder and starting it from scratch, it will reach a certain percentage and freeze at scanning transaction history.
The other problem is that i cannot let armory open bitcoind itself, otherwise i get RPC username and password errors. Currently i am running them separately.
I really hope that the newer version runs bitcoind on its own by using cookie like you mentioned in your posts.
Thanks