Early on, wasn't there actually word that Magni was having a minor surgery? I've thought their situations might actually be separate for a while, but it's still very odd all around. Though, as it does drag on, I do think it actually bodes well for them coming back.
That said, Vesper could probably spin any story about what happened and it'd be believable. But the one, sadder, thought I've had is that something happened to a family member of Vesper's and that's the reason for the sudden stop and complete radio silence. It would generally fit with some of the ways they've handled things on the JP side when a family member has been ill. We normally have only found out later, right? (I'm think Ayame and Mel.)
If that Rrat holds weight, it might also explain the double announcement. Magni was going to take a short break, Vesper had that emergency issue, so the collab got cancelled and Cover just put out a double note when it was originally just going to be Magni?