After I did the normal apt-get update && apt-get dist-upgrade and reboot, arachni doesn't work anymore. I received the following error message while starting arachni (equal if I start it with sudo or without).

cohst@kali-linux:~$ arachni -h
Traceback (most recent call last):
6: from ./arachni:10:in <main>' 5: from ./arachni:10:inrequire_relative'
4: from /usr/share/arachni/ui/cli/framework.rb:9:in <top (required)>' 3: from /usr/share/arachni/ui/cli/framework.rb:9:inrequire_relative'
2: from /usr/share/arachni/lib/arachni.rb:10:in <top (required)>' 1: from /usr/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:inrequire'
/usr/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in require': cannot load such file -- bundler/setup (LoadError) cohst@kali-linux:~$ sudo arachni -h [sudo] password for cohst: Traceback (most recent call last): 6: from ./arachni:10:in<main>'
5: from ./arachni:10:in require_relative' 4: from /usr/share/arachni/ui/cli/framework.rb:9:in<top (required)>'
3: from /usr/share/arachni/ui/cli/framework.rb:9:in require_relative' 2: from /usr/share/arachni/lib/arachni.rb:10:in<top (required)>'
1: from /usr/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in require' /usr/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:inrequire': cannot load such file -- bundler/setup (LoadError)
cohst@kali-linux:~$

Seems to be a problem with the rubygems.