This is typical with Agfa folders. A green grease polymerizes on the focusing threads and the lens sticks. Go here: (there was a url link here which no longer exists) for a discussion.
First, remove the lens from the shutter.
You can soak in alcohol or acetone, although I apparently got lucky with a 2 day soak in acetone which didn't damage my lens. Tom Hoskinson has warned me that he's seen some lenses damaged by soaking the entire lens. Some people use drops of alcohol applied directly to the threads, reapplied periodically over time until it loosens up. In the thread above I think Tom Hoskinson gives a source for some wrenches that can be used to loosen the lens.
After separation, lubrication, and reinstallation, you'll need to focus the front element at infinity. I used a 6x9 ground glass made by Satinsnow for the purpose (I have several folders), but frosted cellophane tape or waxed paper stretched tight across the film plane will also work with a loupe to set infinity focus. After you're there, reinstall and tighten the focusing ring scale with infinity aligned to the focus mark. BTW, you shouldn't have to remove the set screws on the focus scale ring all the way on disassembly.
Jurgen Kreckel says nearly all Agfa folders need new bellows. He'll do new bellows and a CLA on the shutter and lens (including focus lube and reset) at reasonable cost. He's certo6 on ebay, and has a web site that will show up in google.
Lee