Error: ! Font OT1/mathkerncmss/m/sl/10.95=mathkerncmssi10 at 10.95pt not loadable: Met ric (TFM) file not found

This is a packaging error in miktex. The sansmathaccent package is missing the tfm and vf files (https://ctan.org/tex-archive/fonts/sansmathaccent/tfm, https://ctan.org/tex-archive/fonts/sansmathaccent/vf).

I made a bug report

https://github.com/MiKTeX/miktex-packaging/issues/162

The issue should be resolved fast.

I had the same problem. I uninstalled sansmathaccent and clicked "no" when asked to install. Now compiles fine.

Edit 2020-02-07:

An update to sansmathaccent package is now available (version packaged on 2020-02-06) that solves the problem.

Installing packages on the fly no longer interferes with using Beamer’s default math font.

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Not a full answer, but a MWE demonstrating errors with console output and log excerpt as requested in comments.

Used most recent MiKTeX release on Windows 10 as of 2020-02-05T17:38:51Z.

As mentioned by Simon, uninstalling the sansmathaccent package and preventing MiKTeX from reinstalling it seems to serve as a workaround.

MWE:

\documentclass{beamer}

%\usepackage{siunitx} % Triggers the error.

\begin{document}

    \begin{frame}

        %\url{url} % Triggers the error.

        Text.

        $1+1$ % Triggers the error.

    \end{frame}

\end{document}

Messages:

Process started: pdflatex.exe -synctex=1 -interaction=nonstopmode "Document".tex

Sorry, but miktex-makemf did not succeed. The log file hopefully contains the information to get MiKTeX going again: C:\Users\‹username›\AppData\Local\MiKTeX\2.9\miktex\log\miktex-makemf.log

Couldn't open `mathkerncmssi.cfg' hbf2gf (CJK ver. 4.8.4)

Sorry, but miktex-maketfm did not succeed. The log file hopefully contains the information to get MiKTeX going again: C:\Users\‹username›\AppData\Local\MiKTeX\2.9\miktex\log\miktex-maketfm.log

Sorry, but miktex-makemf did not succeed. The log file hopefully contains the information to get MiKTeX going again: C:\Users\‹username›\AppData\Local\MiKTeX\2.9\miktex\log\miktex-makemf.log

Couldn't open `mathkerncmss.cfg' hbf2gf (CJK ver. 4.8.4)

Sorry, but miktex-maketfm did not succeed. The log file hopefully contains the information to get MiKTeX going again: C:\Users\‹username›\AppData\Local\MiKTeX\2.9\miktex\log\miktex-maketfm.log

Sorry, but miktex-makemf did not succeed. The log file hopefully contains the information to get MiKTeX going again: C:\Users\‹username›\AppData\Local\MiKTeX\2.9\miktex\log\miktex-makemf.log

Couldn't open `mathkerncmss.cfg' hbf2gf (CJK ver. 4.8.4)

Sorry, but miktex-maketfm did not succeed. The log file hopefully contains the information to get MiKTeX going again: C:\Users\‹username›\AppData\Local\MiKTeX\2.9\miktex\log\miktex-maketfm.log

Process exited with errors

Log (only parts relating to errors):

…

LaTeX Font Info:    Redeclaring symbol font `pureletters' on input line 5.
LaTeX Font Info:    Overwriting symbol font `pureletters' in version `normal'
(Font)                  OT1/cmss/m/it --> OT1/mathkerncmss/m/sl on input line 5.
LaTeX Font Info:    Overwriting symbol font `pureletters' in version `bold'
(Font)                  OT1/cmss/b/it --> OT1/mathkerncmss/m/sl on input line 5.
LaTeX Font Info:    Overwriting symbol font `pureletters' in version `bold'
(Font)                  OT1/mathkerncmss/m/sl --> OT1/mathkerncmss/bx/sl on input line 5.

…

("C:\Users\‹username›\AppData\Local\Programs\MiKTeX 2.9\tex/latex/amsfonts\umsb.fd"
File: umsb.fd 2013/01/14 v3.01 AMS symbols B)
LaTeX Font Info:    Trying to load font information for OT1+mathkerncmss on input line 15.

("C:\Users\‹username›\AppData\Local\Programs\MiKTeX 2.9\tex/latex/sansmathaccent\ot1mathkerncmss.fd"
File: ot1mathkerncmss.fd 2020/01/31 Fontinst v1.933 font definitions for OT1/mathkerncmss.)
! Font OT1/mathkerncmss/m/sl/10.95=mathkerncmssi10 at 10.95pt not loadable: Metric (TFM) file not found.
<to be read again> 
                   relax 
l.15  \end{frame}

I wasn't able to read the size data for this font,
so I will ignore the font specification.
[Wizards can fix TFM files using TFtoPL/PLtoTF.]
You might try inserting a different font spec;
e.g., type `I\font<same font id>=<substitute font name>'.

! Font OT1/mathkerncmss/m/sl/8=mathkerncmssi8 at 8.0pt not loadable: Metric (TFM) file not found.
<to be read again> 
                   relax 
l.15  \end{frame}

I wasn't able to read the size data for this font,
so I will ignore the font specification.
[Wizards can fix TFM files using TFtoPL/PLtoTF.]
You might try inserting a different font spec;
e.g., type `I\font<same font id>=<substitute font name>'.

! Font OT1/mathkerncmss/m/sl/6=mathkerncmssi8 at 6.0pt not loadable: Metric (TFM) file not found.
<to be read again> 
                   relax 
l.15  \end{frame}

I wasn't able to read the size data for this font,
so I will ignore the font specification.
[Wizards can fix TFM files using TFtoPL/PLtoTF.]
You might try inserting a different font spec;
e.g., type `I\font<same font id>=<substitute font name>'.

…