I checked on the plugin, but I needed to do a quick debugging as well and it turned out that some values on the options page were not consistent with the database. In this case the best and safest option is to start over, so I deleted the search you have configured – sorry about that – and created a new one. You probably will re-configure the search, I hope it won't happen again. If so, then it's definitely an unknown bug.
You can actually change the grouping header on the advanced options page. Under the “Group results by” option there is an input field where you will see the following text: “Posts from: %GROUP%”. Change it to whatever you want, and it should show on the frontend. Currently the isotopic layout is used so I turned it off as it only applies to vertical results.
I have also applied a few custom css rules, which you can see under the “Theme options->Custom CSS” panel, because some rules were conflicting from the theme. You should make a copy of that text in case you create a new search.Best,
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