If you could please raise a bug report at
http://calibre2opds.myjetbrains.com/youtrack/issues and attach the following:
- The metadata.db file from your calibre library
- the log file from your last run
then I can look to see if I can reproduce your results (and thus work out the fix).
I do not use ratings in my own library, so I do not have a none-trivial library that has meaningful rating values in it to test this feature against. I am guessing that there is some fault that is data related and with an example to find it I can easily fix it when found.