Directory Utility - Error connecting to Active Directory for user accounts

This error is what you usually see when connectivity to the AD domain is lost and can't be re-established. Reasons for this are typically DNS related. I would start there first. Hopefully the AD domain is not based around .local for the TLD? If it is then that's probably where the root of the problem probably lies. Using .local is not recommended however it can work depending on whether the domain has two or more names. For example: dc1.myaddomain.biz.local is OK whereas dc1.biz.local is not. Until that is addressed then losing 'sight' of AD will happen intermittently along the lines you're seeing. That includes 'it' working fine for a period of time and then, all of a sudden, 'it' not. Obviously changing the domain is not a 'trivial' thing to attempt when AD is concerned, so this may be something you have to learn to live with? Large AD infrastructures with many OUs and thousands of Users/Groups can also cause this 'loss of sight'. There may be other reasons such as an intermittent problem with the physical structure of your network; failing or faulty ethernet cable and/or switch or port on a switch. Even a power spike or dip can potentially cause the problem. In rare cases an over-enthusiastic network administrator can inadvertently cause the problem.

Disconnecting from AD and then re-connecting again usually 'cures' the problem.

HTH?