Hi Humayun,
One issue is the version 15.x wasn't certified on AIX 5.1:
Did you rebuild your application with the new v15.5 libraries?
I believe the names changed as well (libxxx to libsybxxx).
Are you using tcp or tli protocol?
Older clients used tli in the interfaces file and using tcp could help resolve this.
I believe this was when the packetsize changed from 512 to 2048 as the default.
(-A will not allow you to use 512 packet size for 15.x and above)
Could that be causing any issues on your O/S or a different change allow this to go through now?
The isql connection difference is odd.
You didn't show the sourcing for v15, but assuming you did the same as v12.5 ./source SYBASE.csh.
It is a bummer we won't know what resolved this.
Regards,
Ryan