It is possible to use servers behind reverse proxies via xhr-polling, with some limitations.
As described, Websockets are not part of HTTP protocol, even if first http request looks like http request, actually, it isn't, therefore most known reverse proxies do not support Websockets and drop many important areas of first Websocket request.
If you know that the server is behind reverse proxy, disable Websockets by setting this option in Clients\www\software\html5\settings.js :
disablewebsocket = true;
That will enforce the usage of XHR-polling and avoid time delay when connecting.
If you do not have a file named "settings.js" in the "C:\Program Files (x86)\TSplus\Clients\www\software\html5" folder, then your TSplus version is older and this documentation does not apply. Please update your system first or contact support.
Please note that the usage of XHR-polling is not as stable as Websockets due its connection nature.
The other limitation using Xhr polling via reverse proxy is the poor support on files upload. While working with XHR in direct connection, done for example via Apache reverse proxy, Apache can accidentally interrupt the file's upload.