[sylpheed:37137] Change Key Binding of Short-Cut Key

Hi,
since a few months I cannot change the key-binding of short-cut key as
used to do

https://sylpheed.sraoss.jp/doc/manual/en/sylpheed-18.html

I have been searching for a solution for a while with no success.
Anyone knows where to look? Or just say if it works or not for him/her? 
I'm running Sylpheed 3.7.0 on a pretty standard and up to date Linux
Mint 20.

Thanks in advance,
a.