Can’t send / receive emails (error: usr/local/cpanel/3rdparty/php54/lib/php/Horde/…)

I get this error while trying to send emails via Horde:
usr/local/cpanel/3rdparty/php54/lib/php/Horde/… is not a directory
I can't receive emails either.

See original post by invalid@example.com (qwikad.com)