Ah, the joys of condo conversion! You bought a 'brand new' house, but the plumbing is 20 years old!
A slight bend shouldn't account for such problems. Something is clogging that line. What have they done each time there is backup?
The HOA is responsible for this, although short of legal action, YOUR insurance may be on tap for the damages in your apartment. A stop-gap solution would be to install a back-water valve on the sink drain. This may not be totally effective, as is could cause the backup to go higher, affecting possibly other drains in your apt, or units above.
You need to keep pressure on the HOA to solve this problem. Get a letter from the famous law firm of Dewey, Cheatham, and Howe. Let the condo know you are holding them resposible.