Hi Kevin,

The new 5.1.2 version of the Server includes a new version of the Monitor Console, which, in turn, includes the new 6.1 version of the JavaScript library and the latter upgrade also requires a license upgrade (maybe your customer uses a license updated to 5.1.x server but 6.0.x JavaScript client).
Please get in touch with lightstreamer support and specify the MAC address of the involved license, we can provide you with the upgraded license file.
Alternatively, you can also temporarily replace the ls-monitor.jar file in the 5.1.2 installation with the version of that file that was included in the 5.1.1 installation; the latter should be still compatible with Server 5.1.2.

Regards,
Giuseppe