I got the following error message: "Your order has been rejected due to validation failure. Your session has expired, please relogin." while I was running my strategy in live testing mode.

Why did it happen? Does it mean that all following order submissions will also fail? It happened 3 minutes later after I started the strategy. How can it be handled programmatically? Thanks in advance.

It looks like if it was one of the following:
a) Your account did expire at the very moment.
b) There was some server error which requires reauthorization.

Hm. I'm still able to do historical testing although I've registered half year ago.
The only problem is... there is no exception which can be handled. Should I parse the messages from server? If yes, how can it be done (code sample)?

Currently the only option is filtering DEBUG logs of category com.dukascopy.api.impl.execution by their text contents.