It would probably be better for a dev to answer some of these questions, but I will try my best (as a non-dev).
Strange, I thought BTS has less-than-a minute confirmation time for transaction. But definitely I waited longer for that tx to appear. Could it be some delegate issue? What time does delegate have to produce block and what happens if he fails to do it?
No idea what was going on there. Technically, the delegates could all ignore your transaction and never include it. But if they were doing this intentionally they would eventually be caught and replaced. But in your case, your wallet reports the timestamp of the transaction as 2015-02-14T11:07:50 and the transaction was included in block 1786818 which had a timestamp of 2015-02-14T11:08:00. That is only 10 seconds after which is consistent with the 10 second block interval of BitShares. Maybe your client had network/syncing issues immediately after broadcasting the transaction (as unlikely as it seems)? Or maybe it was some bug in the client.
As for my other block, it was produced with 0.5.3 client and it seems there was some hardfork which prevented 0.5.3. to broadcast transaction. Now I upgraded to 0.6.1 and it seems transactions broadcast OK.
That probably explains it.
I don't quite understand what to do with '521c9756'
Can I somehow cancel it?
Every transaction has an expiration time (after which point the transaction is considered invalid and won't be accepted into the blockchain). By default I believe the client sets the expiration time of transfer transactions to be 1 hour after the transaction was created. So it is safe to assume that this transaction (since it was apparently created on 2015-02-13T08:37:43) is invalid by now. This means even if it was somehow floating around in limbo on the network, it couldn't be included into the blockchain anyway.
I did 'wallet_rescan_blockchain', it took couple of minutes to finish. but 521c9756 still in my transaction history
If i do 'wallet_rebroadcast_transaction' the trnsaction will come to bter? Do I have an option to abandon this concrete transaction? Is it unreversible already? As you told that "block is empty" I consider this transaction "bad" but not "unreversible"
Which command should I use instead of wallet_rescan_blockchain to fix my transact history? I used simple 'rescan' also. No effect. But my GUI crashed after that.
I don't know if the rebroadcast command will update the expiration time, resign, and rebroadcast (in which case the btercom transaction would become valid again) or if it just won't work because it is now expired. Either way, it is best to just give up on the 521c9756 and make a new transfer to btercom.
I am not sure how you are supposed to get rid of the 521c9756 transaction from your transaction history. My guess is that you exported the wallet from v0.5.3 which included the pending transaction 521c9756 and then imported it into v0.6.1, correct? In that case, the client pulled in that now unnecessary (since it wasn't included in the blockchain and has by now expired) transaction into your wallet database (into your transaction history) when you imported the JSON wallet backup. I don't think there is any command to allow you to erase useless items from your transaction history. I think you would either have to just ignore it or perhaps export the wallet again, make a copy of the JSON export, hand edit the JSON in the copy to take out the troublesome transaction, delete the wallets folder, and then import the modified JSON file. But it is probably best to not do that and risk messing up. Hopefully the devs have a better answer.