Hi and thanks for the quick reply!
Sorry, I really totally missed the errors. That is, I initially scanned the log for relevant errors but did (with the original file) not get or not see the one error you mention; I did get and see "warning: ignoring too many clashing note columns" instead, which I rather took as stating the obvious result of what I assumed was a programming error on my behalf (putting some command somewhere when it should be somewhere else). That's why I sort of buried myself in finding something in the notational reference that showed me how/where I should write instead. I have earlier read that the \addlyrics is not meant for complex pieces, yet if it resides inside a monophonic voice variable and works the first time around, who could imagine that this per se is 'complex' and that it stops working the second time?
Anyway, I'll have a closer look at the example snippet you linked and also at the \set associatedVoice command (since most of my pieces have one set of lyrics at the beginning and several at the end of the piece; I did not know of that command until today...).
Thanks again for your help!