networking - Windows server 2008 File Server problems

First of all i would like to apolagies for my lack of knowladge in this matter but with your help i hope i can fix the problem.

Here's the situation:

We have a Windows 2008R2 Server running Databases, File Server, DNS Server and AD controller (I know it is not best pratice, but it's what we got)

Recently, and without any changes in server configuration, some people are having problems accessing their shared folders on the server. When they try to access it says access denied (even when logging on with domain credentials), but what is most weird is that without any changes from my side, people are suddenly allowed to access the folders and after X time the problem returns.... And so on.

I already tried clear all conections with net use *\delete, and I also removed Stored User Names and Passwords (Control Panel\User Acconts\Advanced) but nothing works.

Some times when i try to enter the server address \\192.168.2.100 or \\Umclinica it open the window to enter my credencials and when i try to put the domain credencials for that user it says:

"Logon Unsuccessful: The user name you typed is the same as the user name you logged in with. That user name has already been tried. A domain controller cannot be found to verify that user name."

Another wierd thing i noticed is that until now it only happened on windows XP.

Do u think this might be a Server problem or local PC problem ? I already search all over the place but couldnt figure out the problem.

Beside this problem the server is running as normal, databases are working good, internet (even in the problematic PCs), softwares, etc... everything fine but file server.