Hi @geniusdesign,
The current release of the Old Gregg theme includes JATS Parser library, it can’t be use together with JATS Parser Plugin on the same instance. This will lead to the conflict between libraries and result into a Fatal error. And there is no need to use them both right now - the functionality is the same.
I’m working on a major update to the Old Gregg theme, it’s on the dev branch and nearly half work is done. I’m sorry that it takes so much time, working on it mostly on weekends. It will take another month or so. In this new version the theme will be decoupled from JATS Parser and this problem that you mentioning won’t be relevant anymore.
If you are planning to use JATS Parser Plugin, just remove Old Gregg theme from your themes folder.
This plugin will work with the Classic, Immersion and Health Sciences theme; after the next release, it’s going to be compatible with any OJS theme and will allow showing JATS XML on the article landing page (although it may require some additional styling from a theme side).