Hi, sorry to hear about your issue.
I have accidentally double-posted this thread. Sorry for this, but it does not feel appropriate deleting it now when there is a reply in both. In any case, in the other thread the user "fav" replies:
"you did send the bts and poloniex received it. you don't see it because we currently do not cache the history to this point."
So maybe this is really an issue of block explorer and not the network.
On the matter of Poloniex deposit, I have got a response from support today (3rd day since the issue). They are asking for another deposit of a tiny amount of BTS, in order for them to check the origin. I did as they requested and am waiting for the reply. Will update you here when I have more information.
PS. The response I got reads like this:
From where did you send this? Was it another site? If yes, please send us a screen shot of your withdrawal history on that site containing the transaction. If it was a personal wallet please send exactly X.XXXXXX from the same origin but this time using the unique Memo.ID from your balances page.
I have been using always the same memo (I can even see it in my wallet transaction history), so pretty sure that is not the problem.