When trying to import a .crt file with Kleopatra, I get "The type of this certificate (X.509) is not supported by this Kleopatra installation." This happens with Kleopatra from current kde-neon packages. There are similar closed bugs, like Bug 303790, so it could be a recursion. I cannot provide the .crt file in question unfortunately, but see the same message when trying to open an S/MIME encrypted message in kmail etc. So I am assuming that it is not an issue with this file.
Do you have gpgsm installed? Can you try to import the certificate on the command line with: gpgsm --import <crt-file> ? Thanks
This bug seems to be fixed with new versions of kde (current kde neon user).