Repeating region image upload fails beyond the 13th item

I've come across a strange error whereby in a repeating region with images, item 13 and up does not upload properly.

The repeating region in question is an image gallery with a thumbnail image and main image upload. The gallery uses a plugin called Royalslider, but i don't think this is a contributing factor. You can see a page from the website using this gallery here.

Each item has two image uploads, one for the thumbnail, and one for the full image. (It also has an optional youtube/vimeo url input). I have included the code for this template below.

I start uploading items, selecting a thumbnail image and a main image, and then hitting "Save & add another". I continue doing this for a while, creating more and more items.

It's when I reach item number 13 the trouble starts. As before I add a thumbnail image and a main image and click "Save & add another". The thumbnail uploads fine, and the next item is created, but the main image does not upload.

If i try to upload images for item number 14 or higher, no images are uploaded at all.

In case my explanation isn't making much sense i have created a pdf that visually explains this upload error, you can download it here.

I hope someone can help with this, i need to know why this is happening, and what i can do to fix it.

Thank you

Here is the template code for the image gallery item:

<img class="rsImg" src="<perch:content id="logo1" type="image" label="Gallery Item (Max: 928x650px)" width="928" height="650" crop="true" quality="100" order="2" />" data-rsTmb="<perch:content id="logo2" type="image" label="Gallery Item Thumb (54x54px)" width="54" height="54" crop="true" quality="100" order="1" />" data-rsVideo="<perch:content id="heading" type="text" label="Optional: Vimeo URL (NOT embed code)" required="false" order="3" />" />

And this is the Diagnostics report

Perch: 2.4.5
Production mode: Production (100)
Installed apps: content (2.4.5)
DB driver: PDO
DB tables: perch2_content_index, perch2_content_items, perch2_content_regions, perch2_content_resources, perch2_navigation, perch2_navigation_pages, perch2_page_templates, perch2_pages, perch2_resources, perch2_settings, perch2_user_privileges, perch2_user_role_privileges, perch2_user_roles, perch2_users
Users: 2
App runtimes:
<?php
    include(PERCH_PATH.'/core/apps/content/runtime.php');
?>
Editor plug-ins: markitup
H1: ad59c36f99a07ca5faa474b705995fa9
L1: 1a53a8f4e88b2617f17fff263ad84af4
headerColour: #ffffff
content_singlePageEdit: 1
helpURL:
siteURL: /
hideBranding: 0
content_collapseList: 1
lang: en-gb
update_2.4.5: done
latest_version: 2.8
on_sale_version: 2.8.7
PERCH_DEVELOPMENT: 10
PERCH_STAGING: 50
PERCH_PRODUCTION: 100
PERCH_DB_USERNAME: glory415_user
PERCH_DB_SERVER: localhost
PERCH_DB_DATABASE: glory415_perch
PERCH_DB_PREFIX: perch2_
PERCH_TZ: Europe/London
PERCH_EMAIL_FROM: (removed for privacy)
PERCH_EMAIL_FROM_NAME: (removed for privacy)
PERCH_LOGINPATH: /perch
PERCH_PATH: /home/glory415/public_html/perch
PERCH_CORE: /home/glory415/public_html/perch/core
PERCH_RESFILEPATH: /home/glory415/public_html/perch/resources
PERCH_RESPATH: /perch/resources
PERCH_HTML5: 1
PERCH_ERROR_MODE: DIE
PERCH_DATE_LONG: %d %B %Y
PERCH_DATE_SHORT: %d %b %Y
PERCH_TIME_SHORT: %H:%M
PERCH_TIME_LONG: %H:%M:%S
PERCH_DEBUG:
PERCH_PREVIEW_ARG: preview
PERCH_TEMPLATE_PATH: /home/glory415/public_html/perch/templates
PERCH_DEFAULT_DOC: index.php
PERCH_DEFAULT_EXT: .php
PERCH_PRODUCTION_MODE: 100
PERCH_RWD:
PERCH_HTML_ENTITIES:
PERCH_AUTH_PLUGIN:
PERCH_DB_CHARSET: utf8
PERCH_DB_PORT:
PERCH_DB_SOCKET: