Crystal Reports Ambiguous DLLs issue when upgrading Crystal Reports from v10 to v12 :

Fix:
delete copy of CrystalDecisions.Web.dll v-10.2.51014.0 module
from C:\Inetpub\wwwroot\websitefolder\bin
then:
copy
CrystalDecisions.Web.dll v-12.0.2000.683
from
C:\Program Files\Business Objects\Common\4.0\managed\dotnet2
to
C:\Inetpub\wwwroot\websitefolder\bin