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

[Markus Kohm]

Am Donnerstag, 6. Januar 2022, 14:27:05 CET schrieb Alexander Krumeich:
> I'm sorry I can't provide an MWE here.

Providing a MWE is always possible. It's just a matter of effort. And without
any code shown, every answer would be speculation. Nevertheless I would
speculate that the lines 112 and 116 of one of your TeX files use xpatch to
patch some code of either LaTeX or a class or package and the patch fails
after the update, because the relevant code has changed in LaTeX or the class
or the package.

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