I noticed this was an old problem on FOP1 concerning adobe flash - http://www.asternic.org/archives/msg03086.html
I seem to have the same issue on my FOP2 install, it has been running fine for quite some time (purchased version), all of a sudden the fop webpage loads with:
Couldn't load variables.txt?aldope=64572
The number being different everytime.
I have tried ./fop2_server -test and manager connection is fine, and it starts/stops just fine, so I think it may once again be an incompatibility with flash.
any ideas?