During synthesis, the following error occurs:
"ERROR:HDLParsers:3338 - "<...>" Line 1. Bad file name in project file: Env Variable <...> not defined."
The reason is that the project path contains a "$" character, which is read as an environment variable.
To work around this error, remove the "$" character from the path name.
This issue is fixed in ISE 8.1i Service Pack 3.