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the 5.1.1 file is correct, notice the <type>LICENSE_FILE</type> and that the <client_id> is filled with your client id.
You can use this 5.1.1 file with your 5.1.2 installation

type is wrongly configured:
<type>DEMO</type>

and client id is missing
<client_id></client_id>

The 20 session limit is the limit of the DEMO license: this means that the license was not correctly configured in the new installation.
Please check the conf/lightstreamer_version_conf.xml