Another thing to try, change the filter for that check to the following. Replace everything in $ARG2$ for that service check and see if that helps.
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-a "filter=type in (fixed) and readable = 1 and mounted = 1" exclude="System Reserved" ignore-unreadableIf you see the error again, search in the nsclient.log file for the date and time is happened to see if there is an error.