Maybe an incompatibility with a older/newer node version?
Ensure a node version that is known to build correctly is used, by using a node version manager like [nvm](https://github.com/nvm-sh/nvm).
Requirements for Sage 8.6 (last 8. release):
Node.js >= 6.9, so try building the theme starting with node version 6.9:
$ nvm install 6.9
$ nvm use 6.9
$ gulp --production
When you found a working node version, you may want to add a .nvmrc file to the project with that version to document it and automatically lock that version in when entering the directory, it also helps when deploying with Trellis as it the deploy script can be easily modified to invoke node/npm with nvm and ensure the proper node version is used during the build.