If you run into it more often than during initial installation then you have a problem with one of the hooks that run at
userdata_postsave.
Could you please try the following:
- Disable all other plugins that run at this hook location other than the APTL one
- Create a new user via the AdminCP
- Make a post with this new user in another browser (doesn't matter which)
- Refresh the thread on your main user
If this turns all the posters on that page into guests, please try re-uploading APTL then try step 2 onwards again-
If this does not turn all posters on that page into guests, please re-enable the other plugins one-by-one and follow step 2 onwards until you find the culprit.
When you have found the culprit, open it and paste the code here, and I will fix it for you