"cache" permission

recently i moved to another host.
bak and restor te mysql database.

i got the prolem like below:

when i add some bans there got
"An error was encountered
Error: Unable to write bans cache file to cache directory. Please make sure PHP has write access to the directory 'cache'. "

but when i refresh the bans page,the ban have been added correctly.

my cache is 777,

how about that?