Thanks for your reply Frederick
The WP installation is a single site blog. So i guess W3TC can't be in network mode then. Am i right?
I just installed W3TC and turned on page, object and browser cache.
No other setting that i can remember.
I also got garbled characters for åäö in the vanilla forum install.
So... I just cant figure out why W3TC affects the forum install in the subdomain. Is it possible to exclude the subdomain?
Can i provide you with more info to figure this out?