I had a similar problem where Logic Pro X would not recognize Rex files as valid in EXS24. Couldn't drag and drop either. I found a solution in a terminal command prompt for the Rex Shared Lib (1.7). I downloaded the command prompt here:
Downloaded the setrexfolderspermissions.zip file, unzipped, ran and completed the terminal command.
Viola! Everything worked perfectly in EXS24 like is used to in Logic Pro 9. Does this work for you?