I assume it's a bug in the nostr:npub155m2k8ml8sqn8w4dhh689vdv0t2twa8dgvkpnzfggxf4wfughjsq2cdcvg plugin? Either that or in my nginx redirect rules.
I think your "callback url" implementation is rejecting encoded nostr events which are too long ... these events can get too long (in characters) if there are a lot of relays.... the relevant part of the spec is here: https://github.com/nostr-protocol/nips/blob/3e8c33ee79e3d2eb46ca498eddadcc6ef1ef7329/57.md?plain=1#L31
Discussion
Look at the "protocol flow" here and you'll see actually to pay a zap, you need to make TWO get requests... https://github.com/nostr-protocol/nips/blob/master/57.md