I managed to solve this issue, however I would like to leave this post in case someone else using Cloudflare experiences this problem.
1. What is the issue? Please be detailed.
Without any changes to the ODK Collect or ODK central, on the 7th July 2025 form submissions stopped appearing. On the side of ODK collect it says "Submission Failed" and reports 403 access denied error.
2. What steps can we take to reproduce this issue?
Not possible - it seems to have been an automatic rule change of Cloudflare
3. What have you tried to fix the issue?
I configured a test project with a QR code and after a failed submissions found it in Cloudflare security logs:
Following this I have created a custom rule that is applied first and forces Cloudflare to skip any other firewall rules.
All the best!
Mix