Is there some chance of a sort of 51% attack in that? Could someone with lots of computers convince the network to accept a new version of the account balance tree?
Yes it's called the secret chain attack. Info is on the wiki. It requires someone to maintain 51% for 10,000 blocks and even then it is only able to confuse nodes attempting to sync to the network for the first time. As a general rule, nodes initially syncing to the network should be more diligent in making sure the chain they have received is the one being used by old nodes, such as block explorer etc. However, the attack is of limited utility, since new nodes by definition have no coin, are not exchanges, etc, it would be difficult to gain anything by convincing a new node of a few changes in balances. Old nodes are also able to detect this stuff going on. So we can issue a checkpoint in the code and render worthless all the computing done on the 10000 blocks of 51%.
I second that aizen.
Resync is only possible by a full delete. Had same prob
What OS and version?