First joint of the PVC vent pipe at the furnace wall penetration is always recommended to be sealed with high temperature RTV sealant. This is to ensure that the vent pipe can be easily removed without cutting in case of any furnace servicing.
PVC cement used on this first joint usually fails most likely because of the vibration transferred from the induced draft motor and because two different materials are being jointed together. It seems that PVC cement does not create long lasting air tight joint between the two / it doesn’t melt the cast plastic supplied by the manufacturer.
If you cannot separate this first joint (try to put some pressure on it / move the vent pipe slightly back and forth) there are pretty much three options:
Cut the PVC vent pipe and replace the furnace connector with a new one. However, in order to order a new connector from your local heating supplies store an HVAC contractor’s license might be required (at least that’s the way it is in Illinois). You can try to search for it on-line; some parts for some manufacturers might be available.
Clean the joint area to remove dust / contamination and apply high temperature RTV sealant around the seam. If you ever need to separate this pipe for some reason than you worry about getting a new connector.
Contact the manufacturer and ask them if a rubber sleeve with clamps can be used on this particular furnace model. If so, you could cut this leaking PVC vent pipe at the elbow and re-connect it with a rubber connector.
Please let me know if there's anything else I can help you with.