My site rarely needs to send me an email but I've noticed that when it does, some text seems to be hashed. Here's an invoice for something I bought on my site with Commerce yesterday. There are several yes/no fields which are showing as (identical) hashes rather than the correct "no":

 

Similarly, messages triggered by @Michael.J's Forms app is producing hashes rather than the text content which the recipient is supposed to read:

 

What problems should I take to try to resolve this, please?

The first thing I would suggest is disabling all third party resources and testing without them. There is a high chance one is causing the problem. 

Also revert any custom email templates you have running.