SOLVED - the problem was that I had another content block pair on the same page - All Feeds (Timeline) content blocks. After disabling one of them and resetting the other for all display sizes, my Feed menu works, again.

The new challenge after this experience now will be, how to rearrange content blocks for different display devices when more than one of the same content blocks on the same page causes such difficulties...

Thank you very much for your help, again, LeonidS!