Comment # 5 on bug 1124027 from Wolfgang Bauer

(In reply to Fabian Vogt from comment #4)
> You're using both 42.3 and 15.0 repos - that breaks your system.
> Remove all 42.3 repos, run "zypper dup" and reboot.

But zypper is broken as well, likely because libmodman from Leap 42.3 is
installed.
(In reply to Christopher Bruce from comment #0)
> crb999@linux-ox1l:~> sudo zypper in plasma5-workspace
> [sudo] password for crb999: 
> zypper: symbol lookup error: /usr/lib64/libproxy.so.1: undefined symbol:
> _ZN9libmodman14module_manager8load_dirENSt7__cxx1112basic_stringIcSt11char_tr
> aitsIcESaIcEEEb

Installing the correct libmodman via rpm should hopefully fix that:
sudo rpm -Uvh --oldpackage
https://download.opensuse.org/distribution/leap/15.0/repo/oss/x86_64/libmodman1-2.0.1-lp150.1.6.x86_64.rpm