Sorry for the inconvenience extremely. Can you please let us know, if Is it happening after any recent updates? Please check once by a rollback to the previous version and let us know back kindly.
I can see the error, If the user has added a number to their heading then that heading will not work/jump to the content. But is very easy solve it, simple say to your plugin to read <h2 or <h3 rather <h2> or <h3> You are going to have many incidents because of this and it is very easily solvable.
Thanks for the info. Regarding this, We have already created GitHub ticket in our repository. We will fix this issue and get back to you once it gets fixed.
Here is the reference ticket to track the work updates: https://github.com/ahmedkaludi/Easy-Table-of-Contents/issues/388