From what I can see, there was a breaking ABI change in kcalcore (see https://phabricator.kde.org/D22697 ). This affects KMail (see below).
I guess all packages depending on kcalcore need to be rebuilt.

STEPS TO REPRODUCE
1. Use up-to-date KDE Neon Developer Stable
2. Try to launch kmail

OBSERVED RESULT
kmail: symbol lookup error: /usr/lib/x86_64-linux-gnu/libKF5MessageComposer.so.5: undefined symbol: _ZN8KCalCore9Incidence16staticMetaObjectE

EXPECTED RESULT
(KMail starts)

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.16.80
KDE Frameworks Version: 5.61.0
Qt Version: 5.12.3

ADDITIONAL INFORMATION
$ apt-cache policy libkf5messagecomposer5
libkf5messagecomposer5:
  Installed: 4:19.04.3+p18.04+git20190723.1726-0
  Candidate: 4:19.04.3+p18.04+git20190723.1726-0
  Version table:
 *** 4:19.04.3+p18.04+git20190723.1726-0 500
        500 http://archive.neon.kde.org/dev/unstable bionic/main amd64 Packages
        100 /var/lib/dpkg/status

$ apt-cache policy libkf5calendarcore5
libkf5calendarcore5:
  Installed: 4:19.04.3+p18.04+git20190727.0228-0
  Candidate: 4:19.04.3+p18.04+git20190727.0228-0
  Version table:
 *** 4:19.04.3+p18.04+git20190727.0228-0 500
        500 http://archive.neon.kde.org/dev/unstable bionic/main amd64 Packages
        100 /var/lib/dpkg/status