They added a lot of folder and files that forward us to pop-up windows and russian pages. I talked to the host now. Usually the problem is outdated joomla site or plugins, but I din't think anything was outdated, except a form I used in Iframe from joomla..
We don't know what it is. They couldn't see anything from the log. I will download latest joomla files (done this before...) and move the files I need from the hacked site, make new forms and make a new ftp password.
I'm glad the forum is safe