I have been getting this same error on Synology Docker from time-to-time. The problem will resolve itself, then come back again - sometimes hours, days, or maybe months later. I've confirmed I'm using the "latest" image from DockerHub dated April 28, 2021.
The error causes the Docker container to crash. The container is configured to auto-restart. Restarts are continual up to the maximum restart interval (1 minute).
Log entries show the following "triplet" entries for each restart of the container.
Beginning cache fill
Found 111 channels in lineup, getting channel metadata and guide. This may take a moment.
..Error when filling cache unexpected end of JSON input
The number of "dots" before the word "Error" in the third log entry can vary per occurrence. For example, all the log entries in today's occurrence have 2 "dots". The number of "dots" from a few days ago was different.
I do wish robbiet480 could address this error or provide some sort of recovery in the container.