Bash: syntax error in expression (error token is ...)

This line in a bash file has been working for six months:

SCRATCH_FOLDER_NAME="${SCRATCH_FOLDER_NAME:scratch--folder}"

and today it decided to be no more, with this error:

SCRATCH_FOLDER_NAME: scratch--folder: syntax error in expression (error token is "folder")

What does it mean?

For reference, here is the complete script:

#!/bin/bash

      SHIMMERCAT_SCRATCH_FOLDER_NAME="${SHIMMERCAT_SCRATCH_FOLDER_NAME:shimmercat-scratch--folder}"
REDIS_UNIX_SOCKET="/unpriv/$SHIMMERCAT_SCRATCH_FOLDER_NAME/redis.sock"

if [[ -z ${DONT_RUN_REDIS+x} ]]; then 

    chown shimmercat:shimmercat $SHIMMERCAT_SCRATCH_FOLDER_NAME

    ...
fi