Looks like it's had a bit of water ingress as well on the edge or is starting to.
I pick up used panels from Dumpsters and at the scrap yard all the time. Tell a lie really, I have so many, I don't bother picking them up any more. They are being thrown away all the time these days so just get a replacement panel and be done with it.
Not worth Mucking around trying to fix and old one and although all my system is made from used panels, I wouldn't be putting one that's had the backing compromised and a solder job done on it in an array running 400V @8a or even in a low voltage system I relied on.
Running fans in the shed window is OK but that's all I'd be trusting a repair job to.
Just find a replacement that's been thrown away that's in good nick and you don't have to worry about fixing it or if it will be OK.
I have had a LOT of panels and I have never had one with this problem yet. Might have been badly handled at some time not that the ones I have had were treated like babies.