Rails 2.2.2 session is failing in pstore using file_field_tag

When I try to upload a file using the file_field_tag I get the
following exception (top 10 lines in the stack shown):

/!\ FAILSAFE /!\ Tue Dec 09 17:11:12 -0500 2008
Status: 500 Internal Server Error
End of file reached
/usr/local/lib/jruby-1.1.5/lib/ruby/1.8/pstore.rb:355:in load' /usr/local/lib/jruby-1.1.5/lib/ruby/1.8/pstore.rb:355:inload’
/usr/local/lib/jruby-1.1.5/lib/ruby/1.8/pstore.rb:310:in
transaction' /usr/local/lib/jruby-1.1.5/lib/ruby/gems/1.8/gems/actionpack-2.2.2/ lib/action_controller/cgi_ext/session.rb:48:ininitialize’
/usr/local/lib/jruby-1.1.5/lib/ruby/1.8/cgi/session.rb:273:in
initialize' /usr/local/lib/jruby-1.1.5/lib/ruby/gems/1.8/gems/actionpack-2.2.2/ lib/action_controller/cgi_ext/session.rb:19:ininitialize’
/usr/local/lib/jruby-1.1.5/lib/ruby/gems/1.8/gems/actionpack-2.2.2/
lib/action_controller/cgi_process.rb:94:in session' /usr/local/lib/jruby-1.1.5/lib/ruby/gems/1.8/gems/actionpack-2.2.2/ lib/action_controller/cgi_process.rb:130:instale_session_check!’
/usr/local/lib/jruby-1.1.5/lib/ruby/gems/1.8/gems/actionpack-2.2.2/
lib/action_controller/cgi_process.rb:78:in `session’

The problem appears to be in the temp session file in RAILS/tmp/
session; if I try to load it using Marshal::load(filename) I get a
“Marshal data too short” error. I can store other values in the
session without any problems. Any ideas on what I can do to get
around this?

Rails 2.2.2
Mongrel 1.1.5
JRuby 1.1.5
Ubuntu (hardy)

Much appreciated.