On Premises Data Gateway Registration Connection Errors

When registering a gateway I complete the form asking for the new name and a recovery key. When I click the "Configure" button a few seconds pass and the following error is shown:

The service cannot reach the gateway. You may need to change the account the gateway runs under.
Request error: Id: 'e0c147db-4d32-45a2-8fdc-49f5863dcae8', Status code: 'BadRequest', Timestamp: '06/15/2018 10:27:35 +00:00', ErrorCode: 'DM_GWPipeline_UnknownError', Detail: ''
   at Microsoft.PowerBI.DataMovement.ExternalClient.PowerBIDataMovementClientExtensions.<EnsureSuccessfulDMMSRequest>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.PowerBI.DataMovement.ExternalClient.PowerBIDataMovementClientExtensions.<DiscoverUnifiedGatewayStatusAsync>d__4.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.PowerBI.DataMovement.GatewayCommon.ConfiguratorClientExtensions.<DiscoverUnifiedGatewayStatusAsyncRetry>d__6.MoveNext()

On app.powerbi.com under Manage Gateways the gateway is listed with an error:

Activity ID:	b2ed54f0-1994-40d7-b948-29096c8a0dba
Request ID:	d287b4f9-cf7a-0171-a7df-fd30aa679f23
Cluster URI:	https://wabi-uk-south-redirect.analysis.windows.net
Status code:	400
Error Code:	DM_GWPipeline_UnknownError
Time:	Fri Jun 15 2018 11:28:50 GMT+0100 (BST)
Version:	13.0.5628.141
Underlying error message:	Unable to load DLL 'Microsoft.PowerBI.DataMovement.Pipeline.XPress9.dll': The specified module could not be found. (Exception from HRESULT: 0x8007007E)
DM_ErrorDetailNameCode_UnderlyingHResult:	-2146233052
Troubleshoot connection problems

I have uninstalled and reinstalled the Gateway multiple times. I've resarted the server, I've changed the account the service runs under, I've configured the service to run under HTTPS instead of TCP.

Any help?

Thank you!