I see that this version was sent to you as a temporary fix.
Since then, for Lightstreamer 5.1.x, we have released an official update 6.1.4 of the JavaScript SDK, so an upgrade is recommended.
Note that version 6.1.4 requires Lightstreamer Server 5.1.2, but if you haven't upgraded the Server yet there is no problem, as long as you don't change your javascript code to use some newly introduced API methods.

We cannot expect that the upgrade will fix the issue, but the use of an official release could simplify the investigation.
In fact, we may have to provide you with a new patched library that would help us collecting more information on the conditions in which the exception is throw.