My mistake. I restarted the app too early.
After the full download of almost 8 GB EN n-gram dataset, you need to give your Cloudron instance enough time to unpack and move the files to (by default) /app/data/ngram/en.
In my case, it took more than 15 minutes to meet the requirements.
The moment you restarted the app too early, the zip file was not unzipped completely. In this case the error occurs and you have to start again from the beginning.
Search your logs for ===> en ngram dataset has been installed.