TL;DR: Please try running Memtest86+, this worked for me!

About two or three weeks ago, I was having the worst problems getting past the Sync step. I spent probably about a week or two re-downloading the Bitcoin blockchain (more than once), rebuilding the Armory DB (several times), and rescanning, and it would always fail during the scanning phase with that same "Cannot get tx copy, because don't have full StoredTx!" error often at different points (at different %'s completed).

I became convinced there must be a bug somewhere in Armory. On one occasion in the debug log, it mentioned an additional error message "Invalid txIndex at height # index #". This gave me a place to start looking in the LevelDB database to see if there actually was a corruption. After quite a bit of digging, I did indeed find the issue: in the DB, each transaction in a block is given an ID starting at 0 (that's not the blockchain TxID). There was a transaction in the block whose sequential ID number was greater than the total number of transactions in that block (which is recorded separately in the DB). But how did this happen??

Looking at the transactions that should have been in this block, there was also one missing, so it looked like the corrupted transaction matched up with the missing transaction except for it's sequential ID. Looking closer, the ID was almost correct: it had just a single-bit error in its high nibble, flipping a 0 to a 1 and making it too large.

Well that's strange... maybe my hard drive was failing? Usually HD failures doen't cause single-bit errors, but rather entirely unreadable sectors or relatively large corruptions though.... Maybe a failing RAM stick?

Well, very long story short, I did indeed have some bad RAM which Memtest86+ found pretty quickly. After replacement, I had no trouble rebuilding and rescanning.

A big thanks to Armory for helping to find this bad RAM! If it weren't for all of the double and triple checking in the Armory code, this could have gone undetected for months, and who knows what I could have lost!! Thanks!!!