We could't connect because there are currently no available resources. Try again later or...

LucaBerniniKT Solved the issue on my end!

I checked the status of session hosts in my environment, and noticed something odd.

PS C:\WINDOWS\system32> Get-RdsSessionHost -TenantName contoso.ca -HostPoolName contosoWVD

SessionHostName : contosoWVD-0.contoso.ca
TenantName : contoso.ca
TenantGroupName : Default Tenant Group
HostPoolName : contosoWVD
AllowNewSession : True
Sessions : 0
LastHeartBeat : 7/3/2019 8:41:18 PM
AgentVersion : 1.0.833.5
AssignedUser :
OsVersion : 10.0.17763
SxSStackVersion : rdp-sxs190429002
Status : Available
UpdateState : Succeeded
LastUpdateTime : 7/3/2019 8:38:18 PM
UpdateErrorMessage :

SessionHostName : contosoWVD01-0.contoso.local
TenantName : contoso.ca
TenantGroupName : Default Tenant Group
HostPoolName : contosoWVD
AllowNewSession : True
Sessions : 1
LastHeartBeat : 8/13/2019 12:37:59 PM
AgentVersion : 1.0.1006.2006
AssignedUser :
OsVersion : 10.0.17763
SxSStackVersion : rdp-sxs190614002
Status : Available
UpdateState : Succeeded
LastUpdateTime : 8/6/2019 8:08:11 PM
UpdateErrorMessage :

Our environment only has one session host. The other host "contosoWVD-0.contoso.ca" was probably one of our testing iterations early on. The strange part is that it was marked as "Available" even though it doesn't exist and last heartbeat was 7/3/2019. This must be a new bug starting a few hours ago. After removing the defunct host, it worked again. Traffic must have been routing to the old host in error.

Hope that saves someone the hours I spent on it this morning.

LucaBerniniKT Borked on us too after Windows Updates were installed this afternoon. Get-RdsSessionHost indicates the last heartbeat was received right before the VM was rebooted to install updates. KB4512508 is the culprit - I uninstalled it but the agent was still messed up. Dropped the Session Host from the host pool, uninstalled all the Remote Desktop agent stuff, followed the instructions for registering a session host to an existing pool via powershell, and got it working again after a reboot. Going to pause updates for a bit until a new agent comes out.

LucaBerniniKT I have similar problem and I can see the the agent on the session host is not posting heartbeats properly to WVD they way it used to. Did you resolve your problem? 

LucaBerniniKT Same problem in our environment. Yesterday we were using it, and this morning we are getting this error. Nothing has changed in between. Something down on Microsoft's end?