Over the last 48 hours or so the ESP8266 core has been revised to version 2.6.1. https://github.com/esp8266/Arduino
This was a fairly significant change. For example, the softwareSerial library in the current version is not backward comparable with with earlier releases.
This creates some incompatibilities between the Heltec’s ESP8266 board manager libraries found in \AppData\Local\Arduino15\packages\Heltec-esp8266\hardware\esp8266\0.0.1\libraries and the libraries found in /Documents/Arduino/Libraries.
For instance, the syntax used with the softwareSerial library located in appData is different than the syntax needed for the softwareSerial library in /Documents/Arduino/Libraries.
The Heltec board manager needs to be updated to remove these differences. This is likely to be a big job.