I have did a clean install of vanilla 2.3.1 and i have it http://example.com/subfolder after the installer is finished i go into admin the button uptop says visit site and it brings up the installer again then if i click home again installer pops up
well i am running on 2.3.1 I have tried everything known to man still unable to get this thing to work. it installs without issue but my home button always brings up dashboard setup im running plesk with php 7
I made some progress if i type in http://example.com/subfolder/index.php it works
That sounds more like a browser cache problem. Installation, as far as I have understood, has been finished successfully. If you were able to install, I guess you do not have a .htaccess file problem. Nevertheless you could try if changing the RewriteBase line helps: you would have to delete the # sign (which comments out that line) and make it RewriteBase /forum
But I really think that this is a browser caching problem.