I found a major threading problem in BlogC++'s
NLTemplate templating engine. Now it turned out that the original author (and all forks which might have fixed it) have disappeared from GitHub entirely, for reasons yet unknown.
So I decided that BlogC++ version 9 - to be released some time in 2018, I guess - will come with
inja which accidentally also fixes one of the other problems I had, namely the lack of support for
if blocks in my templates. Hooray! Time to work.