I uninstalled SmartFTP client, deleted the remaining (and probably harmless) HKLM/Software/SmartFTP key, installed the 1.5.991.15 dev version, settings untouched. Added this file to the global queue:
ftp://ftp.nai.com/pub/datfiles/english/readme.txt with default download destination. Set 'On Queue Complete' to 'Stop Queue'. As the download finished, the error appeared. With the 'Idle' action it didn't appear.
Are there perhaps any services required for the q complete actions? My Task Scheduler is active. There were no entries in the event log regarding this error. Would a mem dump help you (I noticed a MINIDUMP folder but that was a long time ago), if so how can I produce it?