I've just upgraded a copy of our forum from 3.4.7 to 4.0.5 by doing the following

1. Cloning DB and files to a new install (on a separate URL)
2. Updating the DB and URL lines in the conf_global.php file
3. Logging into the clone and changing the forum URL and upload directory
4. Copying the files in the 4.0.5 zip over the top of the new forum,
5. Going to /admin/upgrade and following all the prompts

It appears to run this all fine, no issues reported. But when I try and go to the forum now, the CSS and JS files are trying to be loaded from the old domain (where the new files don't exist). So everything is plain HTML.

I can login to both the admin and the community areas of the forum, it's just crippled because the JS is broken.

Did I miss a step? Is there something else I need to do or clear?

Thanks,
Greg

I've have same problem, blank theme without any css / js

 

Only static images are shown

 

The usual cause of this is not changing the upload URL etc in the 3x settings before upgrading but I see you have done that. i think its likely the same issue as this for you (even though some of this relates to a 4.x to 4.x upgrade) the lack of css are the same:

got the same issue. waiting for the support to reply.