I had YAF working on a portal
http://dnn8.us1.glantonsupport.com/aquaebluedemo/  - on page YAF-Forum

I went to add a YAF Forum to a different portal on same instance:
http://dnn8.us1.glantonsupport.com/demo 
- after pressing the Create new Board button, and was prompted to upgrade to latest version, which I did

I then went to add a new Board, but the URL redirects to an invalid one - basically it keeps missing out the "/demo/" part of the portal alias.
By editing the URL each time, I was able to add a new Board.

However, every link that I click to navigate still removes the "/demo/" part of the portal alias.

When I went back to the original Board on the first portal, that has stopped working too for the same reason.

The only way I can get it to work is by installing it on the root portal (i.e. the original one "http://dnn8.us1.glantonsupport.com/" ) - but that is not an option.

I have checked all the configuration settings - web.config / YAF Settings for base URL / portal alias and nothing changes when I modify these
(I have learned that only the portal alias is considered to create the URL - the other settings are not used - despite what the installation notes tell you).

Any ideas on a solution ?

I understand that this is now fixed - thanks to Ingo Herbote for that.
https://github.com/YAFNET/YAFNET-DNN/issues/28 

Is there a release for this fix ?

No that was a different issue. The issue you have is caused because the child port alias is not added to the URL. Im working on a fix soon, then I release an updated version for dnn.

The issue is now fixed in Build 5744

https://github.com/YAFNET/YAFNET-DNN/releases/tag/v2.2.3-5744