I've added some extra checks to avoid this issue in the future.
In the meantime, you can fix this issue by deleting "libplacebo-199-6.cache" in C:\Users\<user>\AppData\Roaming\J River\Media Center 29\Cache\JRVR (it'll be re-created on next launch, no user-data is stored in there)
Can you let me know what size that file has before you delete it? Something got broken in there.