In version 4.1.20-6 of UPPAAL Stratego, when you choose a diagnostic trace of any kind (some, shortest, fastest), you get a prompt with "Bug: Ok expected".
The terminal shows the following stacktrace:
Bug: Ok expected
at on.fU.run(Unknown Source)
I am not sure what query your are trying, but I suspect that it is an SMC query and the reason for this bug is that engine returns a concrete trace but the concrete simulator is broken and thus crash.
We are currently working on the concrete simulator.
If the query is a simbolic one (A, E<> etc), then this is new, and I would like more details (please confirm).
You are right. The problem only occurs for SMC queries.
*** This bug has been marked as a duplicate of bug 643 ***