When generating client proxies from certain WSDLs, including UPS web services, stylesheet generation errors can occur when one of the exception classes is instantiated for the first time.
One workaround is to put a breakpoint in class CL_PROXY_STYLESHEET_TT method TRANSFORM_CREATE_FAULT on the call to method me->transform_element (using SE24). From there you can step into the method and populate variable CV_TT with <<<tt:cond>. This only needs to be done first two times it hits the breakpoint. Once the stylesheet is generated successfully you should not see any more stylesheet generation errors.