No problem for me, too.
I am thinking into the FS_Retail problem.
I tried the Retail FRED and that does not see this problem.
Do you use the old [V] Iceni? I will tell you later why I ask it, I suspect it could be the source of the problem.
SUGGESTION:
Change the faulty SEXP into 'true'.
Save the mission and see if there will be (an) other error(s). If it finds an error, then the mission code is somehow 'screwed up'. (<- Is there a more civil word instead of this?)
If the error goes away, recreate the SEXP without typing in anything manually if possible.
It is not a problem your SEXP tree will be exactly the same, if the code got screwed up, it will fix it.
If this does not work, then at least you tried.