[ucharfli]

Updated system and packages:
# snap -s

 $ sysctl kern.version
                      
kern.version=OpenBSD 6.1-current (GENERIC.MP) #49: Mon Aug  7 22:06:26 MDT 2017
    deraadt@amd64.openbsd.org:/usr/src/sys/arch/amd64/compile/GENERIC.MP

All packages are updated
# pkg_add -uvi
# pkg_delete ncmpcpp
# pkg_add ncmpcpp
Run ncmpcpp:

 $ ncmpcpp
terminating with uncaught exception of type std::runtime_error: ctype_byname<char>::ctype_byname failed to construct for C
[1]    42212 abort      ncmpcpp

I think the problem is special to me(!)