Rule works but produced "err_empty_response" error when I want to edit it So I have created a rule, that works, does what it should exactly do (so the syntax and configuration is okay), but after creating it and waiting a while, the rule becomes un-editable and the edit page gives just WSOP, while disable, export, delete etc still keeps working. I tried manually recreating the rule, it again works and is editable for a while, until it does the same thing.  There is also no error generated on apache when this happens. I also tried it on multiple computers and browsers. I have clean path URL enable though, could that be an issue?
Here is the exported rule code

{ "rules_odeslat_pp" : {
    "LABEL" : "odeslat PP",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules", "mimemail" ],
    "ON" : { "user_update" : [  ] },
    "IF" : [
      { "user_has_role" : {
          "account" : [ "account" ],
          "roles" : { "value" : { "4" : "4", "6" : "6", "3" : "3" } },
          "operation" : "OR"
        }
      },
      { "NOT data_is_empty" : { "data" : [ "account:field-pp-odeslany" ] } }
    ],
    "DO" : [
      { "variable_add" : {
          "USING" : { "type" : "integer", "value" : [ "account:field-stav-uctu" ] },
          "PROVIDE" : { "variable_added" : { "pz" : "PZ" } }
        }
      },
      { "data_calc" : {
          "USING" : {
            "input_1" : [ "pz" ],
            "op" : "-",
            "input_2" : [ "account:field-pp-odeslany" ]
          },
          "PROVIDE" : { "result" : { "ns" : "novy stav" } }
        }
      },
      { "data_set" : { "data" : [ "account:field-stav-uctu" ], "value" : [ "ns" ] } },
      { "data_set" : { "data" : [ "account:field-stav-uctu" ], "value" : [ "ns" ] } },
      { "mimemail" : {
          "key" : "vyplata",
          "to" : [ "account:mail" ],
          "cc" : [ "" ],
          "subject" : "Informace o Vaší odměně",
          "body" : "Važený zpracovateli,\u000D\u000A\u000D\u000Aadministrátor webu právě odeslal [account:field_pp_odeslany] Kč do účtárny! Nový stav vašeho účtu na webu Prostudent: [account:field_stav_uctu] Kč. \u000D\u000A\u000D\u000APokud nebudou peněžní prostředky připsány na Váš účet do 15. dne v měsíci, obraťte se prosím na zákaznickou linku a problém rádi vyřešíme.\u000D\u000A\u000D\u000ATento email je pouze informativní, neodpovídejte na něj.\u0022,",
          "plaintext" : "Važený zpracovateli,\u000D\u000A\u000D\u000Aadministrátor webu právě odeslal [account:field_pp_odeslany] Kč do účtárny! Nový stav vašeho účtu na webu Prostudent: [account:field_stav_uctu] Kč. \u000D\u000A\u000D\u000APokud nebudou peněžní prostředky připsány na Váš účet do 15. dne v měsíci, obraťte se prosím na zákaznickou linku a problém rádi vyřešíme.\u000D\u000A\u000D\u000ATento email je pouze informativní, neodpovídejte na něj.\u0022,",
          "language" : [ "" ]
        }
      },
      { "data_set" : { "data" : [ "account:field-pp-odeslany" ] } }
    ]
  }
}
Have you tried this on multiple servers as well? It is likely something on the server, so other browsers and computer won't make the difference. You did not specify the Drupal version but I ran into a similar situation with Drupal 8 while editing Views.

For me the solution was to increase the PHP memory limit. The linked page shows several ways to do this, but I chose to do it in the php.ini file. I believe it is 256M now.