Joseph Wright wrote a message to 10 package authors last weekend (and I am sure he does not mind quoting it here).
I've done a recent expl3 update in which I've remove functions
deprecated in 2013, following the pattern we've set up. A sweep of TeX
Live shows there are a few stray uses of \int_case:nnn, \tl_case:Nnn,
\str_case:nnn and \str_case_x:nnn, all of which should now end "F"
(e.g. \str_case:nnF). These are easy to miss (I've got one myself to
fix in siunitx), but updates of your code to match expl3 would be
Where I can I've made pull requests on GitHub/BitBucket with fixes.
I've also asked that TL don't update the expl3 version until people
have a chance to address this.
So, MikTeX updated too quickly. Less than a week seems not enough for 10 package authors to update their packages.
Update: I updated mhchem. The upload to CTAN was made 2015-07-23. It will take a day to get approved, a day to distribute to the mirrors and a few extra days until you will eventually see it as a MiKTeX update.
This addendum might be useful for others, but is not related to the mhchem package: After correcting the signature (from
:nnF), the functions were renamed once again to correct the type of expansion they perform.
\str_case_x:nn(TF) is now
x expansion is slightly different from the
e expansion, and the latter is correct for these functions).