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.