Check which features you have installed (via Help > About Tersus Studio > Installation Details > Features).
I'm pretty sure you'll find both a tersus.enterprise and a tersus feature, this means your installation is originally an opensource one, which has been upgraded to an enterprise one, and you'll need to upgrade both at once, by selecting 2.1.18 from both the enterprise-latest and latest update sites.
This can be avoided by installing an enterprise studio from scratch, using an enterprise installer. Contact us for details.
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