It turns out I was *completely* wrong in my assumption about the cause of this error. Sorry.
(Probably better error messages could be helpful here, like: stating *which* argument is invalid, *why* it is invalid, and *what* would be valid.)
It is not the Unicode at all, I am very happy to say.
However, for some reason there are some abbreviation "names" that seem to be invalid. ".al" is one. ".wh" seems to be another. I find it impossible to see a pattern, I can appreviate to two or three characters, even four. It seems abbreviations starting with "." or "," can only be two characters total, except I can do this
:iab ... ellipse
and it works fine.
So what can and can *not* be used for naming abbreviations, when, and *why not*?
The problem also occurs with vim --clean.
I am using Vim 8.2 on Devuan (that's what is in Devuan's repos.)