1. Find the config file of the site and set enablesessionstate = "true ",
<PagesEnablesessionstate= "True"Enableviewstate= "True"Enableviewstatemac= "True"Validaterequest= "False"Pageparserfiltertype= "Microsoft. Sharepoint. applicationruntime. sppageparserfilter, Microsoft. Sharepoint, version = 12.0.0.0, culture = neutral, publickeytoken = 71e9bce111e9429c"Asynctimeout= "7">
2. Cancel the comment of <Add name = "session" type = "system. Web. sessionstate. sessionstatemodule"/>.
< Httpmodules >
< Clear />
< Add Name = "Sprequest" Type = "Microsoft. Sharepoint. applicationruntime. sprequestmodule, Microsoft. Sharepoint, version = 12.0.0.0, culture = neutral, publickeytoken = 71e9bce111e9429c" />
< Add Name = "Outputcache" Type = "System. Web. caching. outputcachemodule" />
< Add Name = "Formsauthentication" Type = "System. Web. Security. formsauthenticationmodule" />
< Add Name = "Urlauthorization" Type = "System. Web. Security. urlauthorizationmodule" />
< Add Name = "Windowsauthentication" Type = "System. Web. Security. windowsauthenticationmodule" />
< Add Name = "Rolemanager" Type = "System. Web. Security. rolemanagermodule" />
< Add Name = "Session" Type = "System. Web. sessionstate. sessionstatemodule" />
</ Httpmodules >