Hi, Shameem -
The issue is most likely related to the large number of libraries in the target's library list. The list of pbl's previously selected in this dialog window is kept in the registry:
Computer\HKEY_CURRENT_USER\SOFTWARE\Sybase\PowerBuilder\21.0\Target\target-drive-and-path-and-name.pbt
String registry values are limited to 2048 characters in length, I believe, so if the list of selected pbl's exceeds this maximum, PB cannot store/recover the value. I cannot say why PB 12.1 is purportedly able to do this successfully, but I suspect it may have kept this information in an .ini file instead of in the Windows Registry, but this is purely a guess on my part as I do not this version available to me.
The number of pbl's you have seems excessive to me, particularly if you only keep a small number of objects in most. You may wish to consider consolidating the contents of some of these libraries into a more manageable quantity.
Best regards, John