Having the same problem tonight. I can generate a BTC address and have successfully deposited in the past. But it won't let me generate one for LTC deposits. I get that exact same error message.
It seems like it is happening to me from today. Registered on bittrex months ago and already made depos and withdraws a lot. I hope they will fix the issue in a short time, actually i'm quite surprised as they are usually precise on telling you about issues occurring on the platform.