I would check "cifs domaininfo" to see if the controller sees the domain... you can also "cifs testdc" to test the domain controller connections. It could be a lot of things... I would also check time but at the same time every day it isn't likely the time is off by over 5 minutes only once a day at the same time every day. Do you have a case open?
Re: CIFS access issues every day at the same time for 15 minutes