I always dry my stainless hangers in the oven between sessions. The 260degrees F for about 5 minutes is all it takes. This time i forgot to take them out from the day prior and my fiance preheated the oven to 450 for some cookies. That solder doesn't hold up too well and i've now got a few holders with detached corners and hinges. Oops
Seems strange, I have never seen a film holder "soft Soldered" all I have ever seen were silver soldered or brazed and your oven should have made them only too hot to touch. I think a bit more investigation should be done in this very unusual case.
One of the currently available solders for stainless steel is a zinc-bismuth alloy that is essentially non-toxic and ought to be pretty nearly inert in photo chemicals (Kodak used to sell a galvanized tank for developing). You should be able to find that in better hardware stores and use it to repair your unsoldered hangers...