This problem had already surfaced several months before the stock market crash, [when commercial banks with deposits of more than $80 million suspended payments].
"When" is clearly anaphoric here since it is the relativised element in the bracketed relative clause. It gets its meaning from the antecedent, the preceding noun phrase, "several months before the stock market crash". "When" is adjunct (adverbial) of time within the relative clause.
We understand that 'commercial banks with deposits of more than $80 million suspended payments several months before the stock market crash'.