>>why the c5 errors will happen only once in a month and not always?
If it's a date format issue, perhaps it literally arises once a month.
In Control Panel\Clock and Region\Region for an affected machine , what's the short date and short time format?
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
