@Joerg_Knoerchen, @scrambler –
Found the bug. Amazing no one has ever found this one before!
The bug involved a problem with using Masonry columns on archive pages. There was an extra </div> being emitted in some cases. I’ll send a fixed version along for you to see. It is hard to know exactly what your options were, but just playing with the various options for masonry columns, it seems to be working properly now.