Comment # 9 on bug 1039471 from Sebastian Ernst

... and I also just pulled in all regular updates on the machine, which
produced the original error. Package versions are identical. 

Things are back to normal, too, except for an odd alert - Thunderbird warned me
while opening a particularly large encrypted pdf attachment (about 7 MByte)
that the attachment appears to be empty. But only a fraction of a second later,
I could not even click on OK in the alert, Okular opened up and presented the
file to me just like expected. I have never encrypted / opened this file on
this particular machine, I am sure, so I'd say it is unlikely that it was
hanging around somewhere in tmp. 

Ignoring this hiccup, it works.