The plugin source code reveals that in certain cases you might end-up with a “Could not clear cache” error even though the required object is purged successfully.
The plugin expects one of the two return codes from Varnish, either “200 OK” or “200 Purged”. If your VCL returns a different message to the PURGE operation, you’ll get this error.
To fix this, you should update your VCL and only issue either “200 OK” or “200 Purged” responses to PURGE requests.
I don’t know if this is your issue, but it certainly caused me to scratch my head.