My KMail 5.2.1 Arch Linux has two accounts configured, one with PGP for business stuff using a special signature (default), the other plain. Now I want to send an email over the private account but didn't find a way to do so. Formerly it was possible to start composition from one of the listed accounts but this doesn't work today. Has anything changed?

I usually configure the identity per-account (it's in the account settings, "Advanced") or per-folder.

I have two identities and associate those with the two accounts.

Can you make an example on what exactly you mean by "Send via"? I have an idea, but I want to make sure I'm not mistaken.

Default account is "Google", which is inserted at from although I started to compose from the private pop3 account ("web.de"). And when I use the send via drop down with "web.de" it still uses Google asks for my pgp password. Replying to a received email works without any issue.

It's in the identity settings ("Account di uscita" in my settings, under the "Advanced" tab):

As I said, account configured per identity.
PS: Workaround is to set the local folder as default. Then it works as known meaning the message is created with the identity/account which is being selected currently. Filed a bug report: https://bugs.kde.org/show_bug.cgi?id=363959