Wonderful and very weird. Surely that must be a bug in the sandboxing implementation. I hope Apple doesn't find out and plug that hole in Mavericks.
Now my publishing works again! Yohoo!
Here's the script that I now got to work as an Applescript publishing step.
I'm sure it could be improved to grab the "current document" from the publishing context but I don't know how that's done.
Code:
tell application "OmniPlan"
my exporttoPdf(front document)
end tell
on exporttoPdf(doc)
tell application "OmniPlan"
set theFile to (file of doc) as string
-- remove extension and directory separator and append .pdf instead
set theFile to ((characters 1 thru -7 of theFile) as string) & ".pdf"
set theFile to POSIX path of theFile
-- for unknown reasons, export from the sandbox only works with POSIX files
export doc to POSIX file theFile as "PDF"
end tell
end exporttoPdf