I am sure David will answer soon
What I see is
Using a (linux) compiled 2.9.8 from 29 May, which is OK. There is a change in the code yesterday
https://github.com/dtschump/gmic which might be a fix. Compiled that and no color-curves preview error. Your version is 4 June, the latest Windows (from
http://www.gmic.eu ) 2.9.8 is 8 June, might be worth an update or wait a bit for the next one.