Spectra Button Block

I am a Family Historian who publishes my research on line through my WordPress Website https://www.audley.one-name.net I am just starting to convert my website from the classic to the block editor using the Spectra Block Plugin.

I have used the ‘Spectra Buttons Block’ to open a pdf file. The button works perfectly BUT when I go back in to edit the page, I get the error message ‘This block contain unexpected or invalid content’.

Can you please explain how I identify this ‘unexpected or invalid content‘ and what it is I need to remove. I find the error message confusing as the button block appears to functions perfectly.

This problem occurs on two of my webpages as follows:

https://audley.one-name.net/adley-variant-of-audley/ On this page there are ‘two buttons’ and the error only occurs on the right hand button.

https://audley.one-name.net/aidley-variant-of-audley/ On this page there is one button and the error occurs on that button.

Many thanks in anticipation of your help, please not I am a family historian and not a website designer so please answer accordingly
Thanks.

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