Looks like you are talking about what Franke calls <i>FR9100 neoprene hose connector</i>. I haven't come across this heater but the connector does look just like a compression-fit 1/4" FIPÂ nut you often find on products with neoprene drinkable water hoses (and a hose clamp to secure the hose). Sold in any hardware store - big box or small - anywhere.
In any event, Franke does have a repair kit just for you. Part number <i>FR9213 - Neoprene Tube Repair Kit</i> . See diagram attached below for the part numbers reference. These are being discontinued as far as can see looking up some supplier sites, but Guillen's appears to still have it:
Unfortunately, there is no product picture to verify positively but the description matches. You may want to give them a call before ordering just to be sure because this isn't the least expensive compression-fit nut I've ever seen.
