Hi,
Possibly the FME home directory path of the previous version (FME 2014 or earlier) is written before the path for FME 2015, in the environment variable "Path" in your computer.
I found a good page describing about environment variables. See here first if you are not familiar with Windows environment variables.
> How to set the path and environment variables in Windows (
http://www.computerhope.com/issues/ch000549.htm)
If you have installed FME 2014 and FME 2015 in the machine, you can see their home directory paths in the "Path" variable, like this.
----
<others>;C:\\Program Files\\FME_2014;
<others>;C:\\Program Files\\FME_2015;
<others>
----
If the path for FME 2014 (or erlier) was written before FME 2015, such an error could occur when running the workspace created with FME 2015.
If so, try editing the "Path" variable so that the path for FME 2015 is located before previous version(s).
Be careful not to destroy individual path strings. I recommend you to save the original string before editing as a backup.
Hope this helps.
Takashi
P.S. If you are using the command prompt to run the bat files, you will have to reboot it after editing the "Path" variable.