Small network (50 users), 2 DCs(both are Global Catalog servers), single office.
First symtom was a couple weeks ago, our Exchange server stopped responding to email. I logged in and noticed some 5719 errors, and also noticed that the mapped file share couldn't be accessed from the Exchange server. I restarted the server and all was back to normal.
Fast forward to last week. I created some new distribution lists on the Exchange 2010 server using the EMC. Once created, I couldn't send email to them, and got an error that the email address did not exist. I also created a test USER in Exchange, which was created fine, and I could see the user in ADUC, but when I clicked on "Member Of" tab, I got an error saying that the membership information could not be found in the global catalog, which led me to check the DCs.
DC2 checks out fine when running dcdiag.
DC1 gives me the following error when running dcdiag, right off the bat:
Ldap search capabality attribute search failed on server. Error -81