The emake client failed to create a “key” file in a cache slot directory. The cache was not updated.
The final portion of the error message is provided by the operating system, and indicates what exactly went wrong.
The emake client maintains an “asset directory”, which provides a memory of past builds to help future builds. Certain subdirectories of the asset directory correspond to cache “slots” used by Parse Avoidance and JobCache.
Because emake wanted to update a cache slot, it created a new temporary directory to represent the new state of that slot. And within that directory emake tried to create a new “key” file that would describe the various factors used to identify that slot: normalized forms of the user environment, working directory, and command line of the command to be entered into the cache. But emake failed to open this file for writing.
The most likely reason would be lack of disk space, but see the trailing portion of the error message for the specific error reported by the operation system.