It's probably due to the recent server migration, that the techies carried out last week.
Until they get it sorted out, a simple workaround (if you can't immediately see a post from the relevant member, so that you can click on their avatar anyway) is to click on
any AB memeber's avatar and then manually change the URL to include the username you actually want to see.
For example, if I click on Emmie's avatar above, I'm taken here:
https://www.theanswerbank.co.uk/members/emmie/
Changing 'emmie' to 'buenchico', and hitting 'Enter' then takes me here:
https://www.theanswerbank.co.uk/members/buenchico/
That works with nearly all AB members' profiles but there are just a few (usually where their username includes a space or similar) where it might not work.