Looks like you might be using a non-paginated request for the new delivery box feature.
This will time out for any accounts with over x number of items waiting to be picked up (possibly 100).
Despite the notes pagination works in the same method as /v2/commerce/transactions/
As per https://github.com/arenanet/api-cdi/pull/514 the API dev is looking to modify the current result to accumulate results based upon item, so that 3 stacks of 250 Copper Ore will result in 1 750 Copper Ore entry (or the more practical liklihood of 2 Copper Ore, 3 Copper Ore, 1 Copper Ore, 8 Copper Ore, etc etc etc etc times 50, becoming a nice and neat 750 Copper Ore).
Even after the potential change pagination may still be necessary for people trading a large variety of items, I sometimes delve in to buying all 76/80 green weapons for instance, giving a large number of different yet similar items.
Checking my secondary accounts, looks like the limit per page is 5000. With the non-paginated request erroring out if there are more.
So I guess the number of accounts affected are very edge cases.
Would be great if it could be checked however.