The version 1.5.x.x is very very old and comes with its specific license code
Version 1.6.x.x has a lot of enhancements (it works on AnyCPU, 32 and 64 bit application, it is faster and it implements several other mechanisms and algorithms) and for this reason it comes with its specific license that is different from the 1.5.x.x one.
If you want to use 1.6.x.x you should upgrade your license to 1.6.x.x by paying the upgrade fee. Please contact us using one of the address in Contacts section and ask for the upgrade to 1.6.x.x