Hi,
Next try. In all modules of lr jpeg previews are used except in develop. In develop first a jpeg is shown, then the raw is converted for the screen by the build in acr.
Two suggestions: 1. Problem with acr cache (not writeable?), full, ... You find the configuration inside prefences.
2. "damaged" prefences file with "freaky" values for acr. Make a backup of your lr config file, delete it and start lr. Lr creates a new, clean prefences file. The prefences file is located inside %appdata%\adobe\lightroom\prefences.