Exchange 2013 DAG Cluster Unable to add second sever

I have two servers (EXCH1 and EXCH2) running Exchange Server 2013 Standard, and a third file server that is a DAG Witness.

The first server is a Windows Server 2012 R2 Standard machine and the second server is a Windows Server 2008 R2 Enterprise machine (on an HP Proliant DL360 G4). Both servers have Client Access and Mailbox Roles on them.

I am able to successfully add only one of the two servers as a member server. When I try to add the second server, (regardless of which one ends up being the second server) it fails. It stalls after being at "Adding Server (EXCH1 or EXCH2) to the cluster for an extended amount of time".. Then an error message displays…

error

A server-side database availability group administrative operation
failed. Error The operation failed. CreateCluster errors may result
from incorrectly configured static addresses. Error: The operation
failed. CreateCluster errors may result from incorrectly configured
static addresses. Error: An error occurred while attempting a cluster
operation. Error: Cluster API '"AddClusterNode() (MaxPercentage=100)
failed with 0x5b4. Error: This operation returned because the timeout
period expired"' failed.. [Server: (SERVER FQDN)

I have taken these additional steps also:

1. Ensured that each server is updated appropriately (Windows Update)
2. Admin account with proper credentials used.
3. IP and NIC settings on each server OK.
4. DNS listings of each host also OK.
5. Exchange Server 2013 running as normal on each server.
6. Cluster Manager service(s) started and running on each server.
7. All Windows Firewall ports added to list, Windows Firewall has even been completely disabled to troubleshoot.

It seems like the two servers don't like talking to each other (regardless of which direction the communication is)