Not so serious
The Fda says:
This meeting has been cancelled to allow time for the
resolution of several outstanding issues. The agency intends to continue
evaluating NDA 22340 and, as needed, may schedule an advisory committee meeting
in the future.
https://www.federalregister.gov/OFRUpload/OFRData/2010-15352_PI.pdf
What does it mean?
If they the issues another meeting will be scheduled.
There's nothing sinister about that.
Maybe fast-track has been too fast for Lux to get everything right.
If there were major flaws the FDA could have simply rejected the NDA .
But the FDA simply grants more time for the resolution of issues.
That has happened many times before.