This is normally caused by Sophos Web Protection preventing the real-time data stream from updating the initial display.
This architectural design ensures that all data is scanned by the virus engines and the client does not receive one byte of data until the Web Protection is sure that there is no unwanted content. However, for "streaming content" like web radio, online TV, YouTube videos, ... this can be a problem, because such streaming content typically needs a very long time from start to finish, or even does not end at all. In these cases, the proxy would need much time or even "forever" to download the data. The user would not be able to see/listen to the video/audio stream.
To resolve this, add the Optymyse Server as an authorized website in your anti-virus policies. This ensures that any web pages hosted on that IP are not subject to the web protection content scanning.
Please note that other web filter/protection products may also cause this issue, if you find one and resolve it in a similar manner, please do share your solution with us so we can add it to this page.