Seems like tonight the server has been a bit choppy.
I'm repeatedly and randomly getting stalls on order.waitForUpdate(IOrder.State.FILLED); on EURUSD and GBPUSD in Historical Test mode.
The code may go through 1, 3 or 6ish orders at random only to stall and never return from the above state.

The system has been working unaltered up untill today.

It may be that an order is rejected by server and never gets filled.
Try using the waitForUpdate with timeout parameter:

order.waitForUpdate(3000, IOrder.State.FILLED); // wait for 3 seconds max

I confirm the problem. Something really weird has been happening recently. I've seen that the trading server took more than 2 minutes to close a Buy Limit or a Sell Limit order (not filled ones, just those in IOrder.OPENED state). Basically, every operation with the order resulted in a timeout in WaitForUpdate() function. I use 5 seconds timeout, and up till 2nd May I've never ever seen a timeout happening. In the last days I've been overwhelmed by panic emails from my EA complaining about those timeouts and inability to trade. The EA was trying to close the order, got timeout, wait a second, retried, etc. After about twenty attempts one of them got through and the order was closed, but then for other 19 of them the server sent messages "Group blah-blah-blah does not exists, please relogin". It looked like the server was overloaded and instead of actually performing what it was instructed to do, just enqueued "close order" messages for a while.
I had those problems on 2nd, 8th and 9th of May (at least).

Additionally, there were a lot of disconnects recently. On 9 May I've seen a situation when the client got disconnected from the trading server, but didn't realize it. It still was showing some opened positions, and everything looked like it was connected. When I manually closed it and reconnected, it showed totally diffirent picture - all positions were closed (funny enough, with profit) and the account balance was different.

Those silent disconnects look weird. It maybe that those timeouts were actually happening because of them, not because of some server-side problem.

I use a dedicated server for trading, with the average ping to your servers of 30ms, so it is highly unlikely that it was a network connectivity issue.

Since I haven't changed my EA and some other customer refers to a very similar problem, I suppose it is something on your side. It maybe that it is caused by the platform update on 1st May, I'm not sure.

There was a release on 9th of May that should have fixed these issues.