A thought on this. It's been confirmed that the obelisks should reset in Solo after a certain amount of time. However, the time has not been confirmed. The longest wait I've heard of was 5 hours. However, there is a natural timer at the ruins in the form of the secondary moon. IIRC this orbits about 3 times a day, and could potentially be connected, thereby setting the reset period at about every 8 hours.
This seems like an excessively long time, but if the time criticality due to the mission is ignored then it doesn't seem far fetched (I'm thinking in terms of a simulation, not a puzzle game here).
One piece of personal experience at the site is that I did not experience the 'mission message appears then disappears' thing until the moon rose, then it happened all the time. I logged out at the time due to frustration, but have been back in since when the moon hasn't been up and am pretty sure I haven't expirenced it again.
(Just to be clear, I have also had: a. Voice alert but no message; b. Message but no voice alert; and c. No message and no voice alert. However these have all happened consistently throughout. It is only the message appearing and disappearing which I have not experienced at any time other than when the moon is up.)
It'd be interesting to know if anyone else has had similar experiences. I'll try to test the hypothesis myself and let everyone know the results.