[gantry] Contentarray particle renders nothing when more than one category given

Dear all,

with the latest Gantry 5 (Helium theme) I’m trying to use the basic contentarray particle on the home page, however, if I enter more than one category in the “Categories” field, it just disappears (on the frontend) and nothing is shown at all.
One category displays fine, any one of those I have as long as only one single category is entered. Two or more - nothing.

I can’t even figure where to start searching for errors.

Any ideas? Much appreciated!

PS. It just renders empty space:

        <div class="g-block size-100">
             <main id="g-mainbar">
                                        <div class="g-grid">                        

        <div class="g-block size-100">
             <div id="contentarray-6519-particle" class="g-content g-particle">            <div class="g-content-array g-grav-pages">


By now, I found out that if you enter more than one category, the contentarray particle only displays articles that are in BOTH categories each, not articles FROM both categories.

E.g. Contentarray set to display categories blog1, blog2
Will display only articles that have

        - blog1
        - blog2

Is this intended behavior?
Is there any easy way to change it to an OR regarding categories?