Do you mean you can successful have html5 based remote-control session but not legacy non-html5 based session on the same device?


What are the brand, model and firmware version of your problematic device(s)? What device platform and enrollment mode are your device in?


Do you have same problem for ALL device models enrolled onto your server?  Of for ALL devices of the SAME model?


What are your server and device agent version and build numbers?  


Have you ever successfully remote control to any devices from this server?   Do you have such problem previously (say in the last 1 or 3 months)?  Have your server/firewall been recently upgraded/reconfigured?  


Have you checked if can be successfully resolved into a valid private or public IP?