digitally sign - sign button no longer working

Hi,
One of our users cannot digitally sign (All tools - use a certificate - Digitally sign) PDF's anymore in Acrobat Reader.

When he clicks the Sign button, that button flashes but nothing else happens, while it's supposed to ask for the certificate password and then save the signed PDF.
This used to work without a problem but suddenly stopped working.
The certificate is in the Windows Certificate store (Current User - Personal)
This is the latest version of Adobe Acrobat Reader (24.005.20320) and the case for any PDF, including one that we just made and that has no protection on it.

What we've tried so far:
1-install the certificate in the Windows certificate store again and set the "use for signing" option in Acrobat
2-uninstall and reinstall Acrobat Reader
3-close Acrobat and delete the following three folders:
C:\%userprofile%\appdata\Local\Adobe\
C:\%userprofile%\appdata\LocalLow\Adobe\
C:\%userprofile%\appdata\Roaming\Adobe\

When he uses the same certificate on a fresh Windows 11 installation + Acrobat reader, then that works.
When he creates a new digital ID (new PCKS#12 digital ID file), then that can be used to sign too.