"! Undefined control sequence. <argument> \fail" after update. But what package?

[Alexander Krumeich]

Hi there,

today I've updated my TeX Live 2021 distribution for the first time in
a couple of months. Of course, that update was massive- Afterwards, my
document stopped working. This is the error in the console, printed
while processing the preamble:

---
(/usr/local/texlive/2021-broken-update/texmf-dist/tex/latex/xpatch/xpatch.sty)
! Undefined control sequence.
<argument> \fail

l.112   {}{\fail}

?
! Undefined control sequence.
<argument> \fail

l.116   {}{\fail}

?
)
---

xpatch hasn't been updated for ages, so that package is unlikely the
culprit. I'm at a loss here.

I extracted the names of all the .sty files from the log. With that
list I diffed those names to the working installation of TeX Live 2021
that's in my backup. Thus it turned out that 26 packages differ
between the working installation (from before the update) and the
broken installation. But which one is broken?

I'm perfectly aware that this error description is far from ideal. I'm
sorry I can't provide an MWE here. Never the less: Is there someone
here who can point me towards a solution, or at least to a better
understanding of what's going on here? Thanks.

Best,

   Alexander