pkg: Cannot solve problem using SAT solver, trying another plan

# pkg upgrade -r FreeBSD -n shows this:
Code:
Updating FreeBSD repository catalogue...
FreeBSD repository is up to date.
FreeBSD is up to date.
Checking for upgrades (4 candidates): 100%
Processing candidates (4 candidates): 100%
Cannot solve problem using SAT solver, trying another plan
Checking integrity... done (0 conflicting)
Your packages are up to date.

What is SAT solver?
What kind of problems are this?
Why cannot the problem be solved?
Which other plans do exist?

After all running the same command shows the same output: The other plan failed too, right?
So what now, how to fix this?
 
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