I had a similar issue a while ago, where a nearly new tank failed. I reported it to formlabs and they were nice enough to send me a new one.
Further investigation showed a pinhole in the upper layer that resin was slowly leaking though, wetting the surfaces together (making them especially clear) and curing hard in some places. Thankfully none made it to the edge or into the printer.
The rep I spoke to said that damage could have occurred from cupping suction, a particularly rough peel, or a bit of debris floating in the tank that got pinched between the model or the build plane and the film surface. Our issue started a few days after a print failure, so it’s possible there was a bit of debris in the resin that I missed. If you transferred the resin from the old tank to the new one, be sure to filter it well.