Pull the seam apart, then clean all the old glue off, scraping and sanding, apply two part hypalon adhesive as per the instructions, that will stop it.
How old? My Serie 1 310 is about 15 years old and is constructed from PVC. Repair kits are available for this material but I have yet to make a really good repair in it, unlike Hypalon which is relatively easy. I had a large tear repaired professionally and it was done to a far better standard than I could have achieved. Get a quote from someone to check if it is worth the hassle.