Cannot modify header information – server cache

It means WordPress is outputting something to the browser too early. The file indicated seems to be your own session file (php stores user sessions in small text files in the tmp directory).

Check first the wp-config.php file. Remove the final closing php tag. Make sure you’re not outputting anything in that file (look for print_r, var_dump, echo, print, some html…).
See the manual for more ideas.

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