Hi

What is the address of the website that you are trying to view in reader mode?

I don't know about OP, but this happens to me on virtually any "article" site (i.e almost any time reader mode is available). For instance, Wikipedia's featured English article of the day: https://en.m.wikipedia.org/wiki/Main_Page

Note: Reader Mode still works (for me) pretty consistently in normal browsing, but produces the above error nearly 100% of the time in private browsing. This started about two or three months ago (for me).

[edited out double post]

For me, if I open a private tab by clicking the new tab plus icon, reader mode will work. If I open a link in a new private tab, switch to that tab, and try reader mode, I get the exact error from op. Only way to fix is to copy the URL and paste it in a fresh tab before triggering reading mode.