When you place a scoreboard display and a clickEvent in one sign, the order of the code seems to matter.
For example:
/give @p sign 1 0 {BlockEntityTag:{Text1:"

{\"text\":\"some text\"}

",Text2:"{\"text\":\"some text\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"/scoreboard players set @p objective2 1\"}}",Text3:"[\"\",{\"score\":

{\"name\":\"dummy_player\",\"objective\":\"objective1 \"}

,\"color\":\"red\"},

{\"text\":\"xtra\",\"color\":\"none\"}

]",Text4:"[\"\",

{\"text\":\"last text\"}

]"}}
worked.
However:
/give @p sign 1 0 {BlockEntityTag:{Text1:"

{\"text\":\"some text\"}

",Text2:"

{\"text\":\"some text\"}

",Text3:"[\"\",{\"score\":

{\"name\":\"dummy_player\",\"objective\":\"objective1 \"}

,\"color\":\"red\"},

{\"text\":\"xtra\",\"color\":\"none\"}

]",Text4:"[\"\",{\"text\":\"last text\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"/scoreboard players set @p objective2 1\"}}]"}}
will display the text along with the scoreboard objective perfectly, but will not run the clickEvent code.