If a cacheMap file is corrupt and contains invalid JSON an attempt to (re)download the file from Synapse raises a JSONDecodeError, aborting the download.
A corrupt cache entry on the local file system could be caused by a number of issues out of the control of the Synapse client (an abruptly halted python process, some activity on the user's disk unrelated to Synapse, etc). We could a cacheMap file file that is not parseable as JSON as if it does not exist, this would allow the download to continue and a fresh cacheMap file would be written.
This was encountered and reported by a forum user. In that case deleting the corrupt cacheMap file allowed them to continue.
The reporting user reported this solution fixed their issue in the linked thread. Closing.