This is the link generated by the pagination (i.e. page 2): http://localhost:5004/admin/end_customers?%23%3CProc%3A0x007fed48a49d80%40%2FUsers%2Fmartin%2F.rvm%2Fgems%2Fruby-2.3.1%2Fgems%2Ffacets-3.1.0%2Flib%2Fcore%2Ffacets%2Fsymbol%2Fcall.rb%3A24%3E=2&%23%3CProc%3A0x007fed494c6ba0%40%2FUsers%2Fmartin%2F.rvm%2Fgems%2Fruby-2.3.1%2Fgems%2Ffacets-3.1.0%2Flib%2Fcore%2Ffacets%2Fsymbol%2Fcall.rb%3A24%3E=2&order=id_desc
Decoded part of the URL ;) (for readability):
<Proc:0x007feb2adc4d90@/Users/martin/.rvm/gems/ruby-2.3.1/gems/facets-3.1.0/lib/core/facets/symbol/call.rb:24>=2&#<Proc:0x007feb35418638@/Users/martin/.rvm/gems/ruby-2.3.1/gems/facets-3.1.0/lib/core/facets/symbol/call.rb:24>=2&order=id_desc
UPDATE
It seems that I didn't clarify my issue enough. The problem is that one of the links generated by ActiveAdmin's pagination (Kaminari) is the one that I pasted here. For some reason it contains that estrange piece of code (or error) than then I decoded manually for readability.
It is happening with all my models, so I think it have to be related with ActiveAdmin or Kaminari.