Rails server doesn't start anymore

Question

I worked on a ruby on rails project for quite some time. Today I wanted to continue but if I try to start the rails server, I just get the following output:

Traceback (most recent call last):
    22: from bin/rails:3:in `<main>'
    21: from bin/rails:3:in `require_relative'
    20: from C:/Sites/dev/TEA/config/boot.rb:3:in `<top (required)>'
    19: from C:/Ruby25-x64/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
    18: from C:/Ruby25-x64/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
    17: from C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/setup.rb:10:in `<top (required)>'
    16: from C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler.rb:101:in `setup'
    15: from C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler.rb:135:in `definition'
    14: from C:/Ruby25-x64/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
    13: from C:/Ruby25-x64/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
    12: from C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/definition.rb:3:in `<top (required)>'
    11: from C:/Ruby25-x64/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
    10: from C:/Ruby25-x64/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
     9: from C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/lockfile_parser.rb:13:in `<top (required)>'
     8: from C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/lockfile_parser.rb:14:in `<module:Bundler>'
     7: from C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/lockfile_parser.rb:122:in `<class:LockfileParser>'
     6: from C:/Ruby25-x64/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
     5: from C:/Ruby25-x64/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
     4: from C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/source/git.rb:6:in `<top (required)>'
     3: from C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/source/git.rb:7:in `<module:Bundler>'
     2: from C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/source/git.rb:8:in `<class:Source>'
     1: from C:/Ruby25-x64/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
C:/Ruby25-x64/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require': C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/source/path.rb:1: Invalid char `\x10' in expression (SyntaxError)
C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/source/path.rb:1: Invalid char `\x10' in expression
[...]
C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/source/path.rb:1: Invalid char `\x10' in expression
[...]
C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/source/path.rb:1: Invalid char `\x1E' in expression
C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/source/path.rb:1: Invalid char `\x1E' in expression
[...]
C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/source/path.rb:1: Invalid char `\x1D' in expression
[...]
C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/source/path.rb:1: Invalid char `\x1E' in expression
[...]
C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/source/path.rb:1: invalid multibyte char (UTF-8)
C:/Ruby25-x64/lib/ruby/gems/2.5.0/gems/bundler-2.0.2/lib/bundler/source/path.rb:1: syntax error, unexpected end-of-input
...FC>Ź������������������������������...
...  

For me this look like some sort of char encoding / language problem but I have no Idea where it came from or how to fix it. It would be great if someone could help me.