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I just had an Error. Suddenly I couldn't use operations like the derivative. There is now just the notice: unknown_outofproc_symbolic_engine_failure.
Maybe you'd like to give us further information.
Which version of Mathcad are you using? Prime or "real" Mathcad 15?
Is the effect reproducible?
What happens if you save the worksheet, close Mathcad and open and reload the sheet. Does the effect show up immediately?
Does it happen after a long session, when working on a very large worksheet?
I just rebooted mathcad and now it luckily works as usual.
Thanks anyway!