No Header Found In Table of Content

I’ve been using qubely for a while now and I like the effort the developers have put into it. Recently, I used Table of Content block in my blog post. I set the Heading tag that I want to be included in TOC. However, after saving the changes, it shows no header found error. I’ve disabled all plugins plus CDN, but still the error shows up on the page.

Regarding PHP version, I am using 8.1 with lightspeed-based server provided by Namecheap.

Is there a solution for this issue or should I wait for an update?

The page I need help with: [log in to see the link]