The following error is either thrown when installing a new Sugar instance or when clearing the cache:
Exception in Controller: exception 'SugarApiExceptionError' with message 'lessc fatal error:<br />load error: failed to find tmp_less_custom_compiler.less'
This means that your custom.less file has a syntax error. Fix any syntax errors and try again.
If your custom.less file is empty, just delete the file, clear the cache, and load the Page again.