Windows 10 clients unable to connect to RemoteApp

Hello,

we are running a RemoteApp environment using RDS Gateway running on Windows Server 2016 Standard. Our problem is that clients running on Windows 10 are not able to connect to RemoteApp at all. After entering the credentials, the login window pops up again after 1 second (it's an endless loop). 

An entry with the Server IP to our internal server name was created in the hosts file. Pinging it also works fine.

RDP Client logs show the following (unfortunately in German):

Fehler10.05.2018 12:11:42Microsoft-Windows-TerminalServices-ClientActiveXCore1033AllgemeinKomponentenname:CClientProxyTransport, :: 'Gateway Error' in CClientProxyTransport::SetErrorStatus at 2853 err=[0x800759d9], Fehlercode:0x800759D9
Fehler10.05.2018 12:11:42Microsoft-Windows-TerminalServices-ClientActiveXCore1033AllgemeinKomponentenname:CClientProxyTransport, :: 'CreateConnection failed' in CClientHTTPProxyTransport::Connect at 1205 err=[0x800759d9], Fehlercode:0x800759D9
Fehler10.05.2018 12:11:42Microsoft-Windows-TerminalServices-ClientActiveXCore1033AllgemeinKomponentenname:CClientProxyTransport, :: 'Gateway connection time out is 90' in CClientHTTPProxyTransport::Connect at 1196 err=[0x800759d9], Fehlercode:0x800759D9
Fehler10.05.2018 12:11:42Microsoft-Windows-TerminalServices-ClientActiveXCore1033AllgemeinKomponentenname:CClientProxyTransport, :: 'm_ClientAdapter->CreateTunnel failed' in CProxyRawTrans::CreateProxyConnection at 2119 err=[0x800759d9], Fehlercode:0x800759D9
Fehler10.05.2018 12:11:42Microsoft-Windows-TerminalServices-ClientActiveXCore1033AllgemeinKomponentenname:CAAClientAdapter, :: 'm_spHelper->ReadCreds failed' in CAAClientAdapter::CreateTunnel at 380 err=[0xffffffff], Fehlercode:0xFFFFFFFF

Clients running on Mac OS are able to connect just fine.

Thanks in advance for any help!