I have an exhaust leak on my '91 Audi 90 Quattro 20V in the location pictured (just before the catalytic converter). Any advice on how to fix it?

 

If its leaking right at the seal I took my 5000 to a local independent muffler shop that replaced the spring-loaded coupler you have with one they make. It's called something like a split coupler or whatever. I think you will need to buy the exhaust sealing ring to place between the cat and the downpipe (hidden by coupler). I bought mine from 034motorsports for about $10.

 

I ended up taking it to a local shopand getting the bad section cut out and a flex pipe put in. It made a world of difference in the sound!