This error exist already in rel 5.1.0. I have waited that you should release a fix in your updates. I can’t remember in which rel it was ok. Your updates are coming so often and I can’t check everything every time.
This is occurring for me also. Every change to the preview settings results in the previous preview being deleted from the directory. In addition, after creating a preview the preview window still indicates “not processed” even though it is. Version 5.04 does not have these issues. I reinstalled version v5.04 as the newer versions are just unusable.