After the latest TeX Live update, I'm getting the following error while compiling my book draft with lualatex:
! Undefined control sequence. \@begindocumenthook ...yrr \cyri \cyrl \cyro \ETC.
I'm not sure what might be the culprit (this is the first time I'm getting this kind of error while working on this document) or even how to track it down. Any advice would be highly appreciated. TIA.
P.S. I understand this has got to do with Russian characters or language use, and I do have some words in Russian in the text, as well as a
declaration in the preamble, but these things did not cause any issues before (and the error does not go away if I remove the
P.P.S. The (almost) full preamble with an empty document body is here. It does reproduce the error with the latest TeX Live update.