OK, I fixed it.

The problem had nothing whatever to do with external repositories. In my internet searches I came across this bug report. As a result I checked out the stats on that empty init script for runmbbservice. It was created on the 22nd of November which coincides (more or less) with my attempt to use a new mobile broadband (mbb) device which is unfortunately not supported out of the box (Huawei E3131). That apparently resulted in the creation of the empty init script.

Solution: as indicated in the bug report, remove the offending runmbbservice script. That's it. The upgrade then went through without errors.

At this stage I can still use my TP Link MA260 mobile broadband when I need such a thing, but I do wonder how I should set up the new device when I get the necessary driver and modeswitch settings. I've never had this particular difficulty with mbb devices before.

Never-the-less, I'll mark this problem as solved.