I get this problem whether I sign in as Admin or as a test user. The permissions in the forum manager are across the board for the admin and start thread and post reply for registered user.

Could you check please the Forum_Users table if you have entries with the same UserID ?

You could run as Host user on SQL:

SELECT * From {objectqualifier}{databaseowner}Forum_Users

Tthen check if you have double entries, if yes delete all double entries using MS SQL Management Studio.

Upgrading the DNN Version could not fix any problems with the Forum module.

I suggest restore Files and Database to the point before you upgrade the Forum Module to 4.5.3, Then check the Forum_Users table, fix it.  Then upgrade to the new module, after you have done a new backup of your files and database.  Thats the methode I suggest to the first users post.