Page Expired error when saving

I'm not sure all of the conditions this might occur. In my case, I have a page with a blueprint that has a list with a lot of fields in it. Once enough items are in the list, the page will no longer save when editing it in normal mode. I can still save the page in expert mode. I had initially had the problem when I had a fieldset inside a list, but I tried it with no fieldset and still experienced the issue.

To recreate, use this blueprint (or something equivalent):

```yaml
title: List Test
extends@: default

form:
fields:
tabs:
type: tabs
fields:
list_test:
type: tab
title: List Test
fields:
header.items:
type: list
label: List
description: A straightforward list to test a potential bug
collapsed: true
fields:
.value1:
type: text
label: "Value 1"
default: 'text'
.value2:
type: toggle
label: "Value 2"
highlight: 1
default: 0
options:
0: Off
1: On
validate:
type: bool
.value3:
type: text
label: "Value 3"
.value4:
type: text
label: "Value 4"
.value5:
type: text
label: "Value 5"
.value6:
type: text
label: "Value 6"
.value7:
type: text
label: "Value 7"
.value8:
type: text
label: "Value 8"
.value9:
type: text
label: "Value 9"
.value10:
type: text
label: "Value 10"
.value11:
type: text
label: "Value 11"
.value12:
type: text
label: "Value 12"
.value13:
type: text
label: "Value 13"
.value14:
type: text
label: "Value 14"
.value15:
type: text
label: "Value 15"
.value16:
type: text
label: "Value 16"
.value17:
type: text
label: "Value 17"
.value18:
type: text
label: "Value 18"
```

Create a new page using the blueprint and add 56 items. Try saving it on normal mode. If it works, add some more items and try saving. 56 should (just barely) cause the issue, though, with this blueprint.