Hello Armin,
I have made a mistake!!
432 - 464 = -32, not positive 32. (I messed up the math!) You know what this means... 
464 bp amplicon |----------------------------------------------|
216 bp read |-------------------->
216 bp read <---------------------|
32 bp gap!!! ^^^
Your reads are not merging because they don't overlap at all, at that length.
If you trim at 260 bp on both forward and reverse, you will get
260+260-464 = 56 bp of overlap
or if you trim at 240 you will get
240+240-464=16 bp of overlap
Got to try some of these settings!
Colin
P.S. This is a reminder to myself to always double check the math 